Organic quinones towards advanced electrochemical energy storage: recent advances and challenges

Cuiping Han *^a, Hongfei Li *^b, Ruiying Shi ^cd, Tengfei Zhang ^a, Jing Tong ^cd, Junqin Li ^a and Baohua Li ^c
aCollege of Materials Science and Engineering, Shenzhen University, Shenzhen Key Laboratory of Special Functional Materials, Shenzhen 518060, China. E-mail: hancuiping06@szu.edu.cn
^bSongshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China. E-mail: hongfeilithu@hotmail.com
^cShenzhen Key Laboratory of Power Battery Safety Research, Shenzhen Geim Graphene Center, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
^dSchool of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China

First published on 25th June 2019

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Cuiping Han

Cuiping Han received her B.S. degree from China University of Petroleum (Huadong) in 2010 and PhD degree from Tsinghua University in 2015 with Prof. Baohua Li. She visited Georgia Institute of Technology during 2014–2015 as a visiting scholar in the group of Prof. Zhiqun Lin. Then she worked as a postdoc with Prof. Ching-Ping Wong at the Chinese University of Hong Kong under the support of the Xiangjiang scholar program during 2015–2017. Now she is a research associate at Shenzhen University. Her research is focused on electrode materials for lithium-ion batteries and supercapacitors and their hybrid devices. She has published 1 book and more than 30 peer-reviewed papers.

Hongfei Li

Hongfei Li obtained his B.S. degree and M.S. degree in materials science and engineering from Central South University and Tsinghua University, respectively. After that, he received his PhD degree from City University of Hong Kong under the supervision of Prof. Chunyi Zhi. Now, he is an assistant professor at Songshan Lake Materials Laboratory. His research focuses on flexible and wearable energy storage devices, aqueous batteries and polymer electrolytes.

1. Introduction

The ever-growing energy demand and the depletion of finite fossil-fuel resources have necessitated the exploration of renewable energy resources, such as wind, solar, tidal, biomass and geothermal energy, which are expected to contribute a great proportion to the total energy supply in the near future.¹ However, these renewable energy resources exist over a broad range of geographical areas and are all inherently intermittent. To facilitate the rapid deployment of renewable energy, efficient energy storage technologies are urgently required.

Rechargeable batteries and supercapacitors (SCs) are two of the most promising energy storage devices due to their notable characteristics such as high energy density, high power density and reasonable cycle life.^2–5 For instance, widespread success has been achieved for lithium ion batteries (LIBs) since their commercialization in the 1990s.⁶ Current commercial LIBs are constructed with LiCoO₂, LiMn₂O₄, LiFePO₄, and LiNi_xCo_yMn_1−x−y on the cathode side and graphite on the anode side.^7–9 The reversible faradaic reaction with Li⁺ ions endows LIBs with high output voltage (3.5 V), high energy density (150–250 W h kg⁻¹) and relatively good cycle stability.¹⁰ Analogously, sodium ion batteries (SIBs), which show battery chemistry resembling that of LIBs, are also extensively studied due to the widespread abundance of sodium resources. The representative cathode materials include layered transition-metal oxides (e.g., Na_xMO₂, M = Co, Mn, Ni, Fe, etc.) and polyanions (NaFePO₄, Na₃V₂(PO₄)₃, Na₂FePO₄F, etc.).¹¹ For the anode, materials such as carbonaceous materials (hard carbon, graphite, etc.), metal oxides/sulphides/selenides/phosphides, alloys and so forth have been extensively developed with varied achievements.¹² In the case of SCs, which are known for their high power density and extremely long cycle life, pseudocapacitive materials such as metal oxides/sulphides/selenides/phosphides are widely explored to enhance the energy density of SCs.^13,14 As seen above, the present rechargeable batteries and SCs require the use of large amounts of transition metal-based inorganic compounds. Though significant progress has been achieved, the high cost and limited reserves of these inorganic transition-metal-based materials cause more and more concern with respect to resources and environmental issues.

Alternatively, organic electrode materials, which consist of inexpensive and sustainable elements such as C, H, O, N and S, provide an excellent opportunity to further improve the existing energy storage technologies.¹⁵ In addition, a substantial part of organic compounds can be directly obtained from natural renewable sources or prepared from their derivatives.¹⁶ Moreover, the structure of organic materials can be designed flexibly, which allows easy tailoring of their physical and electrochemical properties, including specific capacity, voltage, electrical conductivity, solubility, mechanical properties, etc. What is more appealing is that, unlike inorganic materials, organic electrodes are generally not limited by the choice of counter-ions, making them attractive for a variety of energy storage device applications, such as LIBs,¹⁷ SIBs,¹⁸ potassium ion batteries (PIBs),¹⁹ zinc ion batteries (ZIBs),²⁰ magnesium ion batteries (MIBs),²¹ aluminum ion batteries (AIBs),²² SCs,²³ redox flow batteries (RFBs),²⁴ Li–O₂ batteries,²⁵etc.

The study of organic electrode materials started in 1969, when Williams et al.²⁶ made the first attempt to use dichloroisocyanuric acid (DCA) as an active material for primary Li batteries. Since then, a lot of organic structures and redox chemistries have been elucidated, including those of small organic molecules (e.g., quinones and dianhydrides)^27,28 and conjugated polymers (e.g., polypyrrole and polyacetylene).^29,30 Nevertheless, small organic molecules are limited by their high solubility in electrolyte while conductive polymers suffer from their low doping level. In the late 1980s, attention was turned to organosulfur compounds,³¹ but their performances are far from satisfactory. With the great success of inorganic electrode materials in either research or commercialization, organic electrode materials have received much less attention for a long time.

In the past few decades, research on organic electrode materials has been revived due to the increased concern with respect to resources and environmental issues. The reported organic electrode materials can be categorized into conductive polymers,³² organosulfur compounds,^33,34 organic radicals,^35,36 carbonyl compounds³⁷ and other compounds based on CN, CN, NN and multiple carbon bonds (CC and CC).^38,39 On the one hand, the derivatives of these organic electrode materials containing similar redox active centers were particularly explored because of the similar electrochemical activity with possibly better performance. On the other hand, the strategies to enhance the electrochemical performance of the current reported organic electrode materials were extensively studied. For example, conductive polymers, though with high electronic conductivity, generally suffer from two main issues: one is the low available doping level, leading to limited specific capacity. The other is poor cycle stability due to the accumulation of dopants as the cycle progresses.⁴⁰ Strategies including optimization of morphology, copolymerization, and hybridization with a robust substrate were incorporated to tailor the energy storage behavior with varied success.³⁹

Quinones are a class of carbonyl compounds that contain two adjacent (or separated) carbonyl groups in an unsaturated six-member ring structure.⁴¹ They are potentially low cost, sustainable, and high energy density electrode materials due to their large specific capacity and excellent electrochemical reversibility, as well as molecular diversity and structural tailorability. For instance, the simplest member of the quinone family is 1,4-benzoquinone (1,4-BQ), which can accept two electrons to deliver a theoretical specific capacity value as high as 496 mA h g⁻¹ at a redox potential of ∼2.8 V vs. Li/Li⁺,⁴² offering an energy density far exceeding that of conventional inorganic cathode materials, such as LiCo₂O₄ (3.9 V vs. Li/Li⁺, ∼140 mA h g⁻¹)⁴³ and LiFePO₄ (3.45 V vs. Li/Li⁺, ∼170 mA h g⁻¹).⁹ Moreover, quinone compounds have great potential to achieve high-rate capability and long-term cycle stability because of the fast redox kinetics of the quinone group and the stable structure. What's more, quinone-based electrodes are generally not limited by the choice of counter-ions, making them attractive for either univalent Li⁺, Na⁺, K⁺, and H⁺, or divalent Zn²⁺, Mg²⁺, and Ca²⁺, and even multivalent Al³⁺ cation storage, thereby making quinones and their derivatives promising electrode candidates for advanced electrochemical energy storage devices (Fig. 1), including LIBs,⁴⁴ SIBs,⁴⁵ ZIBs,²⁰ PIBs,⁴⁶ MIBs,⁴⁷ SCs,²³ RFBs,⁴⁸etc.

Due to the vital significance and great opportunities in this area, there have been a few excellent reviews and progress reports on organic based electroactive materials. Song et al. and Schon et al. provided a thorough overview on the emerging chemistry of organic electrode materials for energy storage applications in 2013 and 2016, respectively.^49,50 Son's work reviewed the applications of quinones and their derivatives in energy-harvesting and -storage systems, but provided limited details on rechargeable batteries and SCs.⁴¹ Wu et al. reviewed quinone electrode materials for LIBs and SIBs.⁵¹ However, there have been a number of very recent publications on quinone based electrode materials for battery systems beyond Li and Na. Therefore, we mainly focus on quinone based materials in this work, aiming at providing a thorough and state-of-the-art overview of quinone compounds in versatile electrochemical energy storage applications. The emerging application of quinones in different energy storage devices, such as LIBs, SIBs, PIBs, MIBs, and ZIBs as well as SCs, will be comprehensively reviewed and compared. The relationship between the molecular structure and polar groups of quinone molecules with the corresponding energy density, voltage plateau, and specific capacity properties will be elucidated. The strategies to address the tap density, conductivity, and solubility issues of quinones will be summarized, followed by the critical challenges and important future directions for the application of quinone compounds as electroactive materials for advanced electrochemical energy storage devices. We hope that this review will serve as a comprehensive reference to attract more attention to organic quinones and facilitate their practical application.

2. Fundamental chemistry of quinones

2.1 Working principles

The electrochemical reactions in conventional inorganic materials involve the insertion and extraction of guest cations (such as Li⁺) into and out of the host lattice, which are accompanied by valence changes of transition metal cations. In sharp contrast, organic quinone compounds store charge via an ‘ion-coordination’ mechanism where the carbonyl groups are reduced during redox reaction, yielding negatively charged oxygen anions, which are then coordinated by guest cations from the electrolyte.¹⁵ The cations can be ether H⁺ protons, or alkaline metal cations (e.g., Li⁺, Na⁺, and Zn²⁺). Upon reoxidation, the cations are released and the quinones return to their neutral state (Fig. 2). What's more, organic electrodes are generally not limited by the choice of counter-ions, making them attractive for either univalent Li⁺, Na⁺, K⁺, and H⁺, or divalent Zn²⁺, Mg²⁺, and Ca²⁺, and even multivalent Al³⁺ cation storage, thereby endowing quinones and their derivatives with versatile applications.³⁸

2.2 Electrochemical properties

The formidable challenges of current energy storage devices for future electric vehicle applications are the insufficient energy density, power density and cycle stability. The energy density is the product of output voltage and specific capacity. The specific capacity depends on the molecular weight and the number of electrons transferred during the redox reaction. Quinone molecules with lower molecular weight and an increased number of carbonyl groups can achieve high theoretical capacity. Organic materials including quinones show large structural variety, which contributes to easy tailoring of their specific capacity through molecular engineering. For example, the basic 1,4-BQ with a molecular weight of 108.1 g mol⁻¹ can transfer two electrons by the reduction of the two carbonyl groups, giving a theoretical specific capacity value of 496 mA h g⁻¹. By adding two extra carbonyl groups into the aromatic ring of benzoquinone, dilithium rhodizonate (Li₂C₆O₆) with four active sites can deliver a theoretical capacity of 589mA h g⁻¹.⁵² Cyclic ketones (C_nO_n), such as C₆O₆, which are composed of only carbonyl units (without any redundant mass) could exhibit the highest theoretical capacity of 957 mA h g⁻¹ among all carbonyl-based compounds.⁵³ Moreover, compared with 1,4-BQ, the theoretical specific capacities of 1,4-naphthoquinone (1,4-NQ) and 9,10-anthraquinone (9,10-AQ) decrease to 339 mA h g⁻¹ and 258 mA h g⁻¹, respectively, due to the gradually increased molecular weight.

The output voltage of a battery is the potential gap between the cathode and anode. Electrode materials with high redox potentials generally serve as cathodes and materials with low redox potentials are used as anodes. Quinone compounds possess a wide working potential range, leading to possible candidates for both cathodes and anodes. The redox properties of quinone compounds are directly related to their chemical structure and can be tuned through judicious incorporation of appropriate functionalities. Electron-donating and -withdrawing groups are common structural tools for tuning the redox potential towards desired directions (Table 1). Typically, the redox potential decreases with the introduction of electron-donating functionalities, including amino (–NH₂), methyl (Me, –CH₃), methoxy (OMe, –OCH₃), ethyl (Et, –CH₂CH₃), n-propyl (nPr, –(CH₂)₂CH₃), iso-propyl (iPr, –CH(CH₃)₂), n-butyl (nBu, –(CH₂)₃CH₃), iso-butyl (iBu, –CH₂–CH(CH₃)₂), tert-butyl (tBu, –C(CH₃)₃), phenyl (Ph, –C₆H₅), –OLi, –ONa, etc.^42,54 Conversely, the redox potential can be increased by attaching electron-withdrawing functionalities, such as fluoro (–F), chloro (–Cl), bromo (–Br), carboxyl (–COOH), trifluoromethyl (–CF₃), perfluorobutyl (–C(CF₃)₃), perfluorohexyl (–C₆F₁₃), –SO₃Na, etc.⁵⁵ This is because these functionalities have a great influence on the lowest unoccupied molecular orbital (LUMO) level, which determines the reduction potential. Kim et al.⁵⁶ examined the redox properties of seven quinone derivatives using first-principles density functional theory (DFT) calculation. They indicated that the increase in aromaticity decreases the redox potential in the order of 1,4-BQ (3.1 V vs. Li/Li⁺) > 1,4-NQ (∼2.6 V vs. Li/Li⁺) > 9,10-AQ (∼2.2 V vs. Li/Li⁺). The introduced –COOH group increases the redox potential to ∼2.3 V vs. Li/Li⁺ for anthraquinone-2-carboxylic acid. In contrast, the –NH₂ group decreases the redox potential to ∼2.1 V vs. Li/Li⁺ for 2-aminoanthraquinone and ∼2.0 V vs. Li/Li⁺ for 2,6-diaminoanthraquinone (Fig. 3a). It is also noticed that by increasing the number of substituent groups, larger redox potential changes can be achieved. However, the introduction of additional functional groups usually does not contribute to the number of active sites, but leads to increased molecular weight, which will adversely affect the specific capacity.

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To tailor the redox potential without decreasing the theoretical capacity, one feasible strategy is to tune the relative position of carbonyl groups. Miao et al.⁵⁷ elucidated that the relative position of carbonyl groups exerts a pronounced influence on the voltage of quinone electrodes. They investigated the redox potentials of 20 parent quinone isomers, including benzoquinones (BQs), naphthoquinones (NQs), anthraquinone (AQs), and phenanthraquinones (PQs), as shown in Fig. 3b, and revealed that their redox potentials increase in the order of para-quinones < discrete-quinones < ortho-quinones (Fig. 3c). Another approach involves introducing heteroatoms into the molecular structure.^58,59 For example, Liang et al.⁵⁸ prepared three quinone molecules, benzofuro[5,6-b]furan-4,8-dione (BFFD), benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (BDTD) and pyrido[3,4-g]isoquinoline-5,10-dione (PID) (Fig. 3d), which contain electron-withdrawing N, O and S heteroatoms, respectively. These substituents with higher electronegativity than carbon could lower the LUMO energy level, thereby resulting in high voltage. As a consequence, the first reduction potential increases in the order of 1,9-AQ (2.27 V vs. Li/Li⁺) < BDTD (2.52 V vs. Li/Li⁺) < BFFD (2.61 V vs. Li/Li⁺) < PID (2.71 V vs. Li/Li⁺) according to the trend of electronegativity of heteroatoms (Fig. 3e–g).

2.3 Solubility

Though organic quinones demonstrate promising theoretical energy densities, their practical electrochemical performances are affected by many factors, such as physical and electrical properties, electrode constitution and structure, electrolyte, testing parameters, etc. Organic quinones, especially low weight molecules, generally show high solubility in aprotic electrolyte due to strong interaction between quinone molecules and the electrolyte solvents, particularly organic carbonate solvents generally used in LIBs/SIBs. This high solubility causes strong dissolution of active quinones from the solid-state electrode, which leads to low coulombic efficiency and rapid capacity fading of the corresponding batteries and SCs. Many attempts have been made to suppress the dissolution issue, including molecular engineering, electrode design, electrolyte optimization, binder/separator modification, etc., which will be reviewed in detail in the following section. Nevertheless, misfortune may be a blessing in disguise. The soluble nature favors the dissolution of redox-active quinone molecules into various electrolytes for high performance redox flow battery applications, which has been reviewed in detail in Yu's work.⁶⁰

2.4 Tap density

Despite the advantageous gravimetric energy density, organic electrodes, constructed with lightweight elements such as C, O and H, commonly show low tap density, leading to limited volumetric performance of solid-state electrodes. This is, however, of equal importance to gravimetric performance for practical applications. Since this originates intrinsically from the polymer backbone, it is hard to noticeably enhance the tap density of quinone materials though molecular design. But one can try to enhance the overall volumetric performance of the electrode by reducing the amount of inactive components (such as binders, conductive additives, and current collectors) involved during the electrode preparation process, on condition that the electrochemical performance can be well maintained.

2.5 Electrical properties

As just discussed, the practical electrochemical performance is not only affected by physical properties, but also by electrical properties. Efficient electronic and ionic transport are essential to facilitate fast and reversible redox reactions. Unfortunately, quinones commonly suffer from low intrinsic electrical conductivity, which will not only lower the practical specific capacity values, but also restrict the high rate charging/discharge performance. To improve the utilization of electroactive moieties, large amounts of conductive additives are necessary in the electrode preparation process, leading to both decreased gravimetric and volumetric energy density of the whole batteries. Ongoing attempts to increase the electrical conductivity include introducing conductive moieties into the molecular structure^61,62 or immobilizing quinone on a conductive matrix such as graphene,²³ carbon nanotubes (CNTs),⁶³ carbon nanofibers (CNFs),⁶⁴etc.

3. Organic quinones for LIBs

The application of organic quinones as high performance electroactive materials enables access to metal-free, low-cost, and environmentally friendly energy storage systems. The commercial electrode materials for current LIB systems are LiCoO₂, LiMn₂O₄, LiFePO₄, and LiNi_xCo_yMn_1−x−y on the cathode side and graphite on the anode side.^7–9 During charging, Li⁺ ions are deintercalated from the crystal lattice of the oxide cathode materials, pass through the nonaqueous electrolyte to reach the graphite anode and are finally intercalated into graphite. To better satisfy the ever increasing demand for electric vehicles, LIBs with specific energy higher than 350 W h kg⁻¹ at the cell level are urgently needed.⁶⁵ The development of high performance electrode materials is the key to solving this issue. Therefore, in the past few decades, a large number of electrode materials have been explored, including metal oxides (Li₄Ti₅O₁₂, TiO₂, SnO₂, Fe₃O₄, Co₃O₄, ZnFe₂O₄, etc.), alloys (Si, Sn, Ge, etc.), Li metal, and so forth.^5,66–71 Though significant progress has been achieved, the high cost and limited reserves of these inorganic transition-metal-based materials have revived organic electrode materials that consist of inexpensive and sustainable elements.

The history of quinone electrodes can be traced back to 1969, when William et al.²⁶ made the first attempt to use dichloroisocyanuric acid (DCA) as an active material for primary Li batteries. Later, Alt et al.²⁷ explored tetrachloro-p-benzoquinone and tetramethyl-p-benzoquinone as battery cathode materials in H₂SO₄ solution. Both molecules are insoluble and stable in the sulfuric acid solution and show redox potentials at 668 and 478 mV, respectively. Since then, many carbonyl based compounds, including quinones and their derivatives, have been studied as electrode materials for LIBs because of their natural abundance, high energy density, and wide potential range of 1.0–3.1 V vs. Li/Li⁺,⁵¹ as well as structural variety and design flexibility. Moreover, quinone compounds have great potential to achieve high-rate capability and long-term cycle stability because of the fast redox kinetics of the quinone group and the stable structure.⁷²

3.1 Small quinone molecules

To date, many organic quinone molecules and their derivatives have been reported as promising electrode candidates for LIBs, as shown in Fig. 4. The simplest member of the quinone family is 1,4-BQ (1 in Fig. 4), which accepts two electrons to deliver a theoretical specific capacity value as high as 496 mA h g⁻¹ at a redox potential of ∼2.8 V vs. Li/Li⁺,⁴² offering an energy density far exceeding that of conventional inorganic cathode materials, such as LiCo₂O₄ (3.9 V vs. Li/Li⁺, ∼140 mA h g⁻¹)⁴³ and LiFePO₄ (3.45 V vs. Li/Li⁺, ∼170 mA h g⁻¹).⁹ However, 1,4-BQ shows remarkable capacity decay due to strong dissolution into the electrolyte. Since the redox properties of quinone compounds are directly related to their chemical structure, the substituents on the BQ skeleton exert a pronounced influence on the redox potential, specific capacity, cycle stability, solubility, and electrical conductivity of the quinone molecules. Sieuw et al. investigated the electrochemistry of 2,5-diamino-1,4-benzoquinone (DABQ) (2 in Fig. 4).⁷³ They found that the H-bonding between the polarized amino group and the carbonyl group leads to a much lower solubility of DABQ than 1,4-BQ in both carbonate and glyme based organic electrolytes. The measured solubility of DABQ was approximately 200 mg L⁻¹ in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 v/v) electrolyte for both LiPF₆ and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salts, and about 90 mg L⁻¹ for 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) (1:1 v/v) and tetraethylene glycol dimethyl ether (TEGDME)/DOL (1:1 v/v) electrolytes at 1 M LiTFSI. In contrast, 1,4-BQ displayed a solubility of about 200 g L⁻¹ in (1:1 v/v) electrolyte. With DME/DOL and TEGDME/DOL electrolytes, DABQ yielded an initial capacity close to the theoretical value of 388 mA h g⁻¹ and exhibited much lower capacity loss during cycling.

In addition to solubility, the substituent groups can also affect the redox potential through adjusting the LUMO energy. Generally, BQs bearing electron-donating methyl (–CH₃) and methoxy (–OCH₃) groups show decreased discharge potential (i.e., 2.8 V vs. Li/Li⁺ for 1,4-BQ,⁴² 2.7 V vs. Li/Li⁺ for 2,5-dimethyl-1,4-benzoquinone (CH₃-BQ, 3 in Fig. 4) and 2.6 V vs. Li/Li⁺ for 2,5-dimethoxy-1,4-benzoquinone (DMBQ, 4 in Fig. 4)).⁵⁴ In contrast, electron-deficient BQs bearing perfluoroalkyl groups, namely 2,5-bis(trifluoromethyl)-1,4-benzoquinone (CF₃-BQ, 5 in Fig. 4), 2,5-bis(perfluorobutyl)-1,4-benzoquinone (Rf₄-BQ, 6 in Fig. 4), and 2,5-bis(perfluorohexyl)-3,6-dichloro-1,4-benzoquinone (Rf₆-Cl-BQ, 7 in Fig. 4), demonstrate an elevated discharge potential of 3.0–3.1 V vs. Li/Li⁺.⁵⁵ Moreover, the perfluoroalkyl groups in the BQ molecules could lower the lipophobicity, which would suppress their dissolution in aprotic electrolyte. As a consequence, the capacity retention of perfluoroalkylated BQs is enhanced compared with CH₃-BQ (55%, 50%, and 37% after 20 cycles for Rf₆-Cl-BQ, Rf₄-BQ and CF₃-BQ, respectively).⁵⁵ However, the introduced inactive moieties significantly decrease the theoretical capacity. For example, the theoretical capacity of C₄F₉-BQ is 99 mA h g⁻¹, which is much lower than that of 1,4-BQ (496 mA h g⁻¹). In a follow-up study, Yokoji et al.⁴² focused on the dimerization of the BQ skeleton, which has minimal influence on the theoretical capacity. 2,2-Bis-p-benzoquinone (BBQ, 8 in Fig. 4) and 1,4,5,8-naphthodiquinone (NDQ, 9 in Fig. 4) were synthesized, and they can accept four electrons to afford high capacity values of 570 and 501 mA h g⁻¹, respectively. LIBs manufactured with BBQ, NDQ, and 1,4-BQ as cathode materials revealed initial discharge capacities of 326, 347, and 157 mA h g⁻¹ at 2.8, 1.8–3.4 (the first plateau: 3.4 V), and 2.9 V vs. Li/Li⁺, respectively. Besides, solubility tests indicated that BBQ and NDQ are less soluble than BQ in EC and diethyl carbonate (DEC) solution mixture, most likely due to increased molecular weight. BBQ with a high initial capacity of 326 mA h g⁻¹ and residue capacity of 170 mA h g⁻¹ after 20 cycles demonstrated the most promising application for LIBs.

In addition to BQs, AQs, NQs and other low molecular weight quinones such as 1,4-NQ (10 in Fig. 4) are also being explored.^73–75 Yao et al.⁷⁵ compared the electrochemical performance of 9,10-AQ (11 in Fig. 4) and 5,7,12,14-pentacenetetrone (PT, 12 in Fig. 4) as cathode materials for LIBs. The 9,10-AQ electrode showed a discharge plateau at around 2.1 V vs. Li/Li⁺ and an initial discharge capacity of 217 mA h g⁻¹, which is 84% of the theoretical value of 257 mA h g⁻¹. However, the capacity of 9,10-AQ quickly decreased to 49 mA h g⁻¹ after 100 cycles due to the strong dissolution of 9,10-AQ into the electrolyte. In the case of PT, it showed step-wise plateau voltage at around 2.5, 2.3, and 1.8 V vs. Li/Li⁺. A higher initial discharge capacity of 236 mA h g⁻¹ was observed for PT, which is 74% of the theoretical value of 317 mA h g⁻¹, considering that all four CO sites can be reduced. In the subsequent cycle performance test, PT retained a capacity of 183 mA h g⁻¹ after 100 cycles, showing a longer cycle-life than AQ. This signifies that the expansion of the π-system effectively works to reduce the solubility. On further introducing S into the PT molecule, the obtained dibenzo-[b,i]thianthrene-5,7,12,14-tetraone (DTT, 13 in Fig. 4) shows increased discharge potentials because of the reduced LUMO energy due to the added electron-withdrawing S heteroatom.⁷⁶

Luo et al.⁷⁷ reported another benzoquinone based polymer, 2,3,5,6-tetraphthalimido-1,4-benzoquinone (TPB, 14 in Fig. 4), with four rigid aromatic groups coordinated to a benzoquinone skeleton, which not only lowers the LUMO energy, but also provides more active carbonyl sites with an extended conjugated structure (Fig. 5a). Due to the four rigid phthalimide groups, the TPB was insoluble in aprotic electrolytes and displayed two voltage plateaus at 3.05 V and 2.02 V vs. Li/Li⁺ (Fig. 5a and c). Moreover, the TPB demonstrated a high initial discharge capacity of 223 mA h g⁻¹ at 0.2C, along with high rate capability (155 mA h g⁻¹ at 10C) and excellent long-term cycling stability (91.4% capacity retention over 100 cycles at 0.2C) (Fig. 5b and d). It was found that six lithium ions bind/unbind reversibly during the discharge/charge process because of a redox couple, Li₆TPB/TPB (Fig. 5a).

Though planar quinone molecules could afford high theoretical capacities by combining a large number of active sites and low molecular weight, the densely incorporated carbonyl groups usually lead to a low unitization rate as well as low practical capacity. This encourages the design of organic molecules with a 3D structured covalent framework. The mostly studied 3D structured quinone molecules are macrocyclic calixarenes and pillararenes bearing four or five BQ units linked by methylene bridges at para positions, respectively.^78–80 These macrocyclic compounds could realize high utilization of active sites because their unique open bowl structure spatially separates the redox-active sites, which not only minimizes the repulsive interactions between the reduced active sites but also allows favorable access for Li⁺ ions.⁸¹ For example, calix[4]quinone (C4Q, 15 in Fig. 4) with eight carbonyls can theoretically afford a capacity as high as 446 mA h g⁻¹. Practically, it exhibits a high utilization efficiency of active sites as evidenced by a high initial discharge capacity of 422 mA h g⁻¹ at an average voltage of 2.64 V vs. Li/Li⁺, together with a high coulombic efficiency of 99% during the whole cycling test.⁷⁸ The use of pillar[n]quinone (n = 4, 5, 6) as an active material for LIBs was also explored.⁸² Pillar[5]quinone (P5Q, 16 in Fig. 4) with both a pillared architecture and a larger cavity than that of C4Q favors Li uptake. As a consequence, P5Q⁸⁰ exhibited an average potential of 2.6 V vs. Li/Li⁺ and an initial capacity as high as 418 mA h g⁻¹ in solid polymer electrolytes, corresponding to 93.7% of its theoretical capacity (446 mA h g⁻¹). Though satisfactory capacity values can be achieved, the detailed redox-reaction mechanism associated with the multiple active sites of the calix[n]quinone and pillar[n]quinone remains unclear. A preliminary study done by Cheng et al.⁸³ revealed the multi-step reduction mechanism of P5Q and elucidated that the uptake of the first five electrons follows a 2–1–2 pattern.

Inspired by macrocyclic multicarbonyl compounds, Kwon et al.⁸¹ designed a 3D structured triptycene tribenzoquinone (TT, 17 in Fig. 4) molecule bearing three benzoquinone units in a rigid tripod structure, which can offer a high theoretical capacity of 467 mA h g⁻¹ if all six CO groups can be reduced. Practically, the TT achieved a specific capacity of 387 mA h g⁻¹ with the initial discharge/charge plateaus at 2.90/2.96 V vs. Li/Li⁺ under 0.1C.

Nevertheless, the successful application of the above small quinone molecules is greatly plagued by two major issues: one is the high solubility of quinone molecules and/or their reduction products in aprotic organic electrolyte, which causes serious capacity degradation in a short time. For example, 1,4-BQ displays a high solubility of about 200 g L⁻¹ in TEGDME/DOL (1:1 v/v) solution.⁷³ The other issue is their low electrical conductivity, resulting in limited practical capacity and poor high rate charging/discharging performance. To address the aforementioned challenges, several approaches have been reported such as conversion of small quinone molecules into their polymer and/or salt forms, immobilization of quinones on conductive carbon (graphene, CNT, and porous carbon) matrices, electrolyte optimization, and binder modification, which are discussed in detail in the following section.

3.2 Quinone polymers

To address the dissolution issue, the structural diversity of organic materials triggers the development of organic electrodes with intrinsic insolubility or slight solubility. In the past few years, successful examples were reported for organic polymers and organic salts, such as polyimides,²² Li₄C₈H₂O₆,⁸⁴ dilithium terephthalate (Li₂C₈H₄O₄),⁸⁵ and azobenzene-4,4′-dicarboxylic acid lithium salt (ADALS).¹⁷ In the case of quinone polymers, most of the reported ones are main-chain type polymers, which are prepared using a polycondensation method. By using 1,4-dichloroanthraquinone (1,4-DCAQ) and 1,5-dichloroanthraquinone (1,5-DCAQ) as the monomers, Song et al.⁸⁶ successfully synthesized three kinds of polyanthraquinone polymers, which are poly(anthraquinonyl sulfide) (PAQS, 1 in Fig. 6), poly(1,4-anthraquinone) (P14AQ, 2 in Fig. 6) and poly(1,5-anthraquinone) (P15AQ, 3 in Fig. 6), respectively. When applied as cathode materials for LIBs, the PAQS, P14AQ and P15AQ showed an average discharge voltage of 2.14 V, 2.14 V and 2.09 V vs. Li/Li⁺, respectively. A solubility test indicated that both the charge and discharge products of P14AQ are insoluble in an electrolyte containing 1 M LiTFSI in a mixture of DOL and DME (2:1, v/v) due to a high molecular weight of ca. 230000. Nevertheless, the discharge products of P15AQ showed strong dissolution behavior owing to a low molecular weight of ca. 2300. Impressively, the P14AQ demonstrated a high reversible capacity of 263 mA h g⁻¹ at 0.2C, and retained 98, 96, 91, 84, 78, and 69% of its initial capacity at 0.5, 1, 2, 5, 10, and 20C, respectively, demonstrating excellent fast discharge/charge ability. The P14AQ also achieved very good cycling stability. After 1000 cycles at 1C and 2C, the capacity retention was 98.1% and 99.4% with a coulombic efficiency of 99.8% throughout the cycle.

Generally, organic polymers are prepared by linking the monomer units with various organic groups, such as thioether (–S–),^72,87,88 methylene (–CH₂–),^78,89 imine (–NH–),⁶¹ and ether (–O–) groups, or grafting on a skeleton chain.⁹⁰ Unraveling the structure–electrochemical property relationship of quinone polymers with their monomer units and the linking bonds is of great significance for the development of high-performance polymer electrodes. Vlad et al.⁶¹ described the chemical synthesis of poly(2,5-dihydroxyaniline) (PDHA, 4 in Fig. 6) with intrinsic electrical conduction and a theoretical capacity of 443 mA h g⁻¹. PDHA combines the redox properties of quinone groups with the electrical conduction of p-conjugation in polyaniline. PDHA displayed a pair of redox peaks centered at 2.3 V and 2.6 V vs. Li/Li⁺ with an initial discharge capacity of 270 mA h g⁻¹. Unfortunately, the cycle stability needs further improvement.

Recently, quinone polymers linked with a thioether bond (C–S–C) have attracted considerable attention. Song et al.⁷² reported the lithium salt of poly(2,5-dihydroxy-p-benzoquinonyl sulfide) (Li₂PDHBQS, 5 in Fig. 6) by linking the monomer units with a thioether bond. The combined advantages of both the O⋯Li⋯O coordination and increased molecular weight make the as-prepared Li₂PDHBQS completely insoluble in LIB electrolyte. Consequently, the Li₂PDHBQS displayed a high reversible capacity of 268 mA h g⁻¹ and a stable capacity retention of 90% after 1500 cycles. In a follow-up report, Song et al.⁸⁷ described the polymerization–oxidation synthesis of poly(benzoquinonyl sulfide) (PBQS, 6 in Fig. 6), which showed a similar structure to that of Li₂PDHBQS, except that the two –OLi groups adjacent to the quinone backbone are eliminated. The elimination of the electron-donating –OLi group can not only elevate the redox potential by ∼0.6 V, but can also largely improve the theoretical capacity from 295 to 388 mA h g⁻¹. Practically, the as-prepared PBQS achieved a reversible specific capacity of 275 mA h g⁻¹ at 50 mA g⁻¹ and stable cycling over 1000 cycles. Similarly, Liu et al.⁹¹ synthesized poly(2,5-dihydroxyl-1,4-benzoquinonyl sulfide) (PDBS, 7 in Fig. 6) by the Phillips method with chloranilic acid and Na₂S·9H₂O. The electrode prepared with 60 wt% PDBS exhibited a reversible charge capacity of 228 mA h g⁻¹ at 15 mA g⁻¹ with well-defined and sloping plateaus in the voltage range of 1.75–2.35 V vs. Li/Li⁺. After 100 cycles, the capacity decreased to 184 mA h g⁻¹, corresponding to a capacity retention of 73.8%.

Apart from Li₂PDHBQS, PBQS and PDBS, other polymers linked with thioether bonds, such as poly(2,3-dithiino-1,4-benzoquinone) (PDB, 8 in Fig. 6)^92,93 and poly(2-chloro-3,5,6-trisulfide-1,4-benzoquinone) (PCTB, 9 in Fig. 6),^94,95 were also investigated as active materials for LIBs. The ladder-structured heterocyclic PDB connected by thioether bonds was examined as both a cathode and an anode material for all-plastic batteries (Fig. 7a).⁹² The electrode was prepared by mixing PDB with carbon nanotubes (CNTs), producing a free-standing architecture with good flexibility (Fig. 7b and c). The electrochemical test revealed that the PDB performed better in ether-based electrolyte than in traditional carbonate electrolyte. When tested as a cathode, PDB delivered specific capacities of 255, 249, 234, 223, 204, 190 and 161 mA h g⁻¹ at 0.02, 0.04, 0.1, 0.3, 0.8, 1 and 1.5 A g⁻¹, respectively (Fig. 7e–g), with 85.9% capacity retention after 500 cycles at 1 A g⁻¹. Moreover, as a proof-of-concept study, an all-plastic battery using pristine PDB as the cathode and prelithiated PDB as the anode was assembled, and it delivered a high energy density of 276 W h kg⁻¹ and a large capacity retention of 119 mA h g⁻¹ after 250 cycles. It is also found that the thioether bond not only ensures the structural stability of the quinone polymer during the redox reactions, but also provides fast electron transfer due to the π-electron delocalization between the lone pair of sulfur and the quinonyl rings.⁹³ The PCTB polymer was prepared by polymerization of chloranil through thioether bonds (C–S–C). The presence of the –Cl electron-withdrawing group endowed the PCTB with high redox potential. The PCTB yielded a discharge capacity of 138.7 mA h g⁻¹ at 30 mA g⁻¹ with an average voltage of 2.72 V vs. Li/Li⁺.⁹⁵

Covalent organic frameworks (COFs) allow precise integration of the redox-active building blocks into two- or three-dimensional (2D or 3D) polymeric frameworks with long-range ordered skeletons and nanopores, and have thus attracted growing interest for energy storage applications. Wang et al.⁹⁶ synthesized a diaminoanthraquinone based COF (DAAQ-TFP-COF) and successfully delaminated the bulk COF into 2D few-layered nanosheets via ball milling (Fig. 8a and b). The exfoliated anthraquinone based COFs (DAAQ-ECOFs) exhibited sheet-like morphology with a thickness of 3–5 nm. Unlike the diffusion-controlled process in the bulk DAAQ-TFP-COF, the reaction kinetics of DAAQ-ECOFs was greatly enhanced due to a shortened ion/electron migration length and large number of Li storage active sites exposed on or near the surface region. When evaluated as a cathode material for LIBs, the DAAQ-ECOF exhibited a reversible capacity of 145 mA h g⁻¹ at 0.02 A g⁻¹, corresponding to 96% of its theoretical capacity. Impressively, DAAQ-ECOF showed a capacity of 107 mA h g⁻¹ and a capacity retention of 98% over 1800 cycles at 0.5 A g⁻¹. Analogously, Lin et al.⁹⁷ reported the synthesis of free-standing single-layer nitrogen-rich graphene-like holey conjugated polymers (NG-HCPs) as an anode for LIBs (Fig. 8c). The 2D NG-HCP nanosheets consist of graphene-like subunits as well as homogeneous hexagonal micropores (ca. 11.65 Å) (Fig. 8d–g). The 2D NG-HCP displayed a very high reversible capacity of 1320 mA h g⁻¹ at 0.02 A g⁻¹ and a long cycle life of >600 cycles. However, the charge storage mechanism is unclear. Similarly, a hypercrosslinked poly-pillar[5]quinone (Poly-P5Q) polymer was successfully synthesized by Ahmad.⁸⁹ Poly-P5Q was prepared by oxidizing a poly-dimethoxypillar[5]arene, which was obtained from the cross-linking of 1,4-dimethoxypillar[5]arene using the methylene (–CH₂–) bond. Though the Poly-P5Q could show an enhanced cycle stability of 82.3% over 100 cycles at 0.1 A g⁻¹, unfortunately, it only delivered a specific capacity of 105 mA h g⁻¹, which is far lower than its theoretical value of 446 mA h g⁻¹.

As discussed above, quinone polymers with both a planar structure and 2D/3D framework structure have been explored. Their high molecular weight offers intrinsic insolubility of polymer electrodes in the electrolyte. Particularly, quinone polymers linked with the thioether bond have attracted a lot of attention in recent studies. Nevertheless, very limited attention was paid to improve the intrinsic electrical conductivity of quinone polymers. Furthermore, the synthesis usually requires a few additional steps. The development of simple and efficient synthetic methods remains challenging. In addition, future studies should pay more attention to unraveling the structure–electrochemical property relationship of quinone polymers with their monomer units and the linking bonds, which is of great significance for the development of high-performance polymer electrodes.

3.3 Quinone salts

The formation of the quinone salt is another approach to solve the solubility problem of small organic molecules. To date, quinone salts with functional groups such as –OLi, –ONa, –COOLi, –COONa, and –SO₃Na have been reported, as shown in Fig. 9. These functional groups show high polarity, thus making the quinone salts less soluble in aprotic electrolyte. Xiang et al.⁹⁸ reported the coordinated polymer dilithiated 2,5-dihydroxy-1,4-benzoquinone ([Li₂(C₆H₂O₄)], 1 in Fig. 9) as a positive electrode for LIBs. A pair of reduction/oxidation peaks at 1.7/2.7 V vs. Li/Li⁺ was observed from the CV profile. A charge–discharge test of the [Li₂(C₆H₂O₄)] based cell demonstrated an initial discharge capacity of 176 mA h g⁻¹ and a coulombic efficiency of 93.18% in the first cycle. The Poizot group has done a lot of work on the lithium salt of quinones. The quinone lithium salt, dilithium rhodizonate (Li₂C₆O₆, 2 in Fig. 9), reported in 2008 (ref. 52) achieved a reversible capacity of 580 mA h g⁻¹ with the first discharge plateau at 2.7 V vs. Li/Li⁺. Unfortunately, a noticeable decrease in the capacity upon cycling was observed. In a follow-up study, Kim et al.⁹⁹ explored the cause of the capacity degradation of dilithium rhodizonate and showed that Li_xC₆O₆ undergo multiple two-phase based transformation processes within 2 ≤ x ≤ 6. The lithiation process leads to particle exfoliation, which can accelerate the dissolution of the electroactive material in the electrolyte. They also suggested that both dissolution and structural changes during lithiation should be considered to improve the cycle stability. Analogously, the Poizot group¹⁰⁰ also reported the lithium salt of tetrahydroxybenzoquinone (Li₄C₆O₆, 3 in Fig. 9), which can be produced either from tetrahydroxybenzoquinone by neutralization at room temperature or via thermal disproportionation reaction of dilithium rhodizonate (Li₂C₆O₆) in an inert atmosphere. The Li₄C₆O₆ compound exhibited a reversible capacity of ∼200 mA h g⁻¹ with a stable capacity retention of 90% after 50 cycles. However, the discharge plateau has been noticeably reduced to 1.8 V vs. Li/Li⁺ compared with the ∼2.8 V vs. Li/Li⁺ of 1,4-BQ⁴² due to too many –OLi groups. Later they reported the quinone salt of lithiated 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone (Li₂DHDMQ, 4 in Fig. 9), which displayed an elevated discharge voltage of 2.5 V vs. Li/Li⁺.¹⁰¹ However, the theoretical capacity is also greatly reduced by the large molecular weight inactive moieties. The residual capacity is only 80 mA h g⁻¹ after 50 cycles at a rate of one Li⁺ exchanged in 2 h.

Apart from the –OLi group, organic materials bearing –COOLi and –COONa groups also show suppressed solubility in aprotic electrolyte. Shimizu et al.¹⁰² studied p- and o-quinone derivatives having two –COOLi groups, namely 2,6-bis(lithiooxycarbonyl)-9,10-anthraquinone (LCAQ, 5 in Fig. 9), 2,7-bis(lithiooxycarbonyl)-9,10-phenanthrenequinone (LCPQ, 6 in Fig. 9), and 2,7-bis(lithiooxycarbonyl)pyrene-4,5,9,10-tetraone (LCPYT, 7 in Fig. 9), whose theoretical capacities are 174, 174, and 296 mA h g⁻¹, respectively. They revealed that the two –COOLi groups significantly enhanced the cycle stability without strongly decreasing the redox potentials compared with their parent quinones. In particular, LCPYT displayed an initial capacity of 217 mA h g⁻¹ with a residual capacity of 187 mA h h⁻¹ after 20 cycles at 0.059 A g⁻¹ (0.2C) at an average discharge plateau of 2.39 V vs. Li/Li⁺. This is comparably higher than that of the parent PYT with a limited initial capacity of 76 mA h g⁻¹ at 2.32 V vs. Li/Li⁺. They also found that the introduction of the –COOLi group decreases the solubility of the LC-quinones in 1 M LiPF₆/propylene carbonate (PC) electrolyte presumably via formation of a network by Li-coordination. Organic tetralithium salts of 2,5-dihydroxyterephthalic acid (Li₄C₈H₂O₆, Li₄DHTPA, 8 in Fig. 9) with two –COOLi groups have also been investigated as active materials.⁸⁴ Li₄DHTPA can serve either as an anode material with redox couples of Li₄C₈H₂O₆/Li₆C₈H₂O₆ at ∼0.8 V vs. Li/Li⁺ or as a cathode material with redox couples of Li₄C₈H₂O₆/Li₂C₈H₂O₆ (9 in Fig. 9) at ∼2.6 V vs. Li/Li⁺. Remarkably, Li₄C₈H₂O₆ nanosheets showed discharge capacities of 223 and 145 mA h g⁻¹ at 0.1 and 5C, respectively, with a capacity retention of 95% after 50 cycles at 0.1C. As a proof of concept, an all-organic LIB with Li₄C₈H₂O₆ nanosheets as both the positive and negative electrode was constructed, and it demonstrated an average voltage of 1.8 V vs. Li/Li⁺ and an energy density of about 130 W h kg⁻¹. Similarly, Renault et al.¹⁰³ explored the dilithium(2,5-dilithium-oxy)-terephthalate salt (Li₄-p-DHT) as a lithiated cathode material.

Other groups such as –SO₃Na have also been reported in organic molecules to suppress the dissolution. Wan et al.¹⁰⁴ investigated the electrochemical performances of anthraquinone compounds with and without –SO₃Na functional groups. Anthraquinone-1-sulfonic acid sodium (AQS, 10 in Fig. 9) and anthraquinone-1,5-disulfonic acid sodium (AQDS, 11 in Fig. 9) salts were studied in comparison with the parent AQ. The average discharge plateau followed the order 2.4 V of AQDS > 2.25 V of AQS > 2.1 V vs. Li/Li⁺ of AQ, confirming again that the –SO₃Na functional group could improve the lithium storage voltage due to an electron withdrawing effect (Fig. 10a). Furthermore, AQDS with two –SO₃Na functional groups showed the best cycle stability, i.e., a retained specific capacity of 120 mA h g⁻¹ after 100 cycles at 0.1C with almost no capacity fading. In contrast, though the parent AQ with no –SO₃Na functional group generated the highest initial discharge capacity of ∼270 mA h g⁻¹, the reversible capacity quickly faded to only 40 mA h g⁻¹ after 100 cycles at 0.1C due to strong dissolution. As can be speculated, the AQS displayed specific capacities and cycle performance between those of AQDS and AQ. Lu et al.¹⁰⁵ reported the incorporation of sodium 1,4-dioxonaphthalene-2-sulfonate (NQS, 12 in Fig. 9) into the network of multiwalled carbon nanotubes (NQS/MWNTs) using a dissolution–recrystallization method, which produced a flexible and free-standing hybrid film for LIB applications (Fig. 10b–e). NQS with a LUMO energy of −3.967 eV exhibited an initial lithiation potential of 2.97 V vs. Li/Li⁺ (Fig. 10c). The average discharge capacities of the NQS/MWNT hybrid film with an average content of 41 wt% NQS were 146, 125, 107, and 93 mA h g⁻¹ at 0.2, 0.5, 1.0, and 2.0C, respectively (based on the mass of NQS), which are considerably higher than that of pristine NQS at the same rates. In addition, because NQS with high polarity exhibited slight dissolution in aprotic organic electrolyte, the NQS/MWNTs delivered an average capacity of 149 mA h g⁻¹ with a high capacity retention of 96% after 50 cycles at 0.2C, which also surpassed the 81% of pristine NQS.

The combined solidification and polymerization could effectively solve the solubility issue of small quinone molecules in aprotic electrolyte. Petronico et al.¹⁰⁶ prepared a lithium salt polymer of dihydroxyanthraquinone (LiDHAQS, 13 in Fig. 9) capable of storing four Li⁺ per monomer unit (Fig. 10f). LiDHAQS delivered specific capacities of 330 mA h g⁻¹ at 0.5C and 350 mA h g⁻¹ at 0.25C with minimal capacity fading over 400 cycles, with two pairs of redox peaks at 2.1/2.3 V vs. Li/Li⁺ and 3.0/3.2 V vs. Li/Li⁺, respectively (Fig. 10g and h).

In summary, the introduction of high polarity groups such as –OLi, –ONa, –COOLi, –COONa, and –SO₃Na groups could effectively suppress the dissolution of quinone molecules in aprotic electrolyte, thus enhancing the cycle stability of the corresponding batteries. Nevertheless, the introduction of inactive moieties with large molecular weight leads to reduced theoretical specific capacity. Moreover, the electron-donating –OLi and –ONa groups will lower the reduction potential, while the –SO₃Na group contributes to an elevated reduction potential due to the electron-withdrawing effect.

3.4 Quinone based composites

3.5 Electrolyte optimization and binder modification

Electrolyte optimization is another effective approach to suppress the solubility issue of quinone compounds. The dissolution of quinones in aprotic electrolyte is mainly caused by their similar polarity to aprotic solvents and/or their nucleophilic properties. Therefore, solvents with weaker polarity and a lower donor number are favorable for alleviating the solubility problem of quinones.¹¹⁸ Quinone compounds generally have much lower solubility in ionic liquids (ILs) and glyme-based electrolyte than in traditional carbonate electrolyte. For example, the measured solubility of DABQ was approximately 200 mg L⁻¹ in EC/DMC (1:1 v/v) electrolyte for both LiPF₆ and LiTFSI salts, and about 90 mg L⁻¹ for DME/DOL (1:1 v/v) and TEGDME/DOL (1:1 v/v) electrolytes at 1 M LiTFSI.⁷³ Similarly, electrochemical tests revealed that the PDB performs better in ether-based electrolyte than in traditional carbonate electrolyte.⁹² Notably, the saturated concentration of C4Q in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([PY13][TFSI]) was 0.42 g L⁻¹ at room temperature, much lower than the 2.05 g L⁻¹ obtained in DME.¹¹⁸ Nevertheless, the high viscosity of ILs leads to lower ionic conductivity, which will limit the fast charging discharging performance.

The solubility problem of quinone compounds can also be effectively mitigated by using quasi-solid-state or all-solid-state electrolytes.^74,119 Gel polymer electrolyte (GPE) is generally prepared by soaking the polymer matrix in liquid electrolyte. Huang et al.⁷⁸ reported a poly(methacrylate) (PMA)/poly(ethylene glycol) (PEG)-based GPE for LIBs with C4Q as the cathode. The PMA/PEG-based GPE soaked with 0.7 mol L⁻¹ LiClO₄ in DMSO solution had an ionic conductivity of 0.57 × 10⁻³ S cm⁻¹. The C4Q cathode with the above GPE delivered a capacity of ca. 380 mA h g⁻¹ after 100 cycles at 0.2C, corresponding to a capacity retention of 89.8%. In contrast, C4Q with 1 M LiPF₆ in EC/DMC (1:1 in vol) electrolyte only preserved approximately 100 mA h g⁻¹ after five cycles owing to C4Q dissolution. To completely remove the liquid components, all-solid-state electrolytes have been reported. The PMA/PEG-LiClO₄-3 wt% SiO₂ solid polymer electrolyte with an optimum ionic conductivity of 2.6 × 10⁻⁴ S cm⁻¹ at room temperature was fabricated by Zhu et al.⁸⁰ The SiO₂ nanoparticles were incorporated as fillers into the polymer matrix to improve the ionic conductivity of the solid polymer electrolyte. The P5Q cathode coupled with this solid polymer electrolyte provided an average voltage of ∼2.6 V vs. Li/Li⁺ and a stable cyclability of 94.7% after 50 cycles at 0.2C. Analogously, Wei et al.⁹⁴ prepared an all-solid-state Li–organic battery with PCTB as the cathode and PEG-LiClO₄-10 wt% Li_0.3La_0.566TiO₃ (LLTO) as the electrolyte. The solid polymer electrolyte with LLTO nanoparticles as nanofillers presented an ionic conductivity of 7.99 × 10⁻⁴ S cm⁻¹ at 343 K. The all-solid-state battery maintained 90% of its maximum capacity (104 mA h g⁻¹) after 300 cycles at 0.06 A g⁻¹ with an average potential of 2.72 V vs. Li/Li⁺ at 343 K. In addition to solid polymer electrolyte, thin films of lithium phosphorous oxynitride (LiPON) solid electrolyte were fabricated by the atomic layer deposition (ALD) method with a remarkably small thickness of 30 nm for all-solid-state thin-film battery applications.¹²⁰ The cathode layer, dilithium-1,4-benzenediolate (Li₂Q), was fabricated using a combined ALD/molecular layer deposition(MLD) approach.

The solid-state ion conductor, either polymer or ceramic electrolyte, can prevent the quinone material and/or its reduction products from passing to the counter electrode, thus effectively improving the cycling performance. However, the low ionic conductivity (e.g., liquid electrolyte: up to 10⁻² S cm⁻¹, GPE: 10⁻⁵ to 10⁻³ S cm⁻¹, Li₇La₃Zr₂O₁₂ ceramic: 10⁻³ to 10⁻⁴ S cm⁻¹, Li_1.5Al_0.5Ge_1.5(PO₄)₃: ∼10⁻⁴ S cm⁻¹) and poor electrode/electrolyte interface associated with the solid-state electrolyte present a great challenge in achieving high rate charging/discharging performance.^121,122

Additionally, the development of multifunctional binders is also beneficial for the performance enhancement of quinone electrodes. For instance, the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was found to have multifunctional roles when coupled with DTT as a cathodic material, which include binder function, conductive additive, and more importantly, a strong interaction with DTT to prevent it from dissolving.⁷⁶ In future, novel binders exhibiting the above-mentioned or more interesting functions, such as stretchability, self-healing, etc., are highly desired.^123–125

4. Organic quinones for SIBs

The limited reserves and uneven global distribution of lithium resources bring fear of lithium shortage, which has stimulated researchers to develop advanced energy storage technologies beyond Li.¹²⁶ The greater natural abundance of sodium compared to lithium and their similar electrochemistry have prompted the development of SIBs.¹²⁷ In recent years, there has been a significant growth in the number of positive and negative electrode materials for SIBs.¹ Layered transition-metal oxides (P2-type Na_xCoO₂ and Na_xMnO₂, O3-type NaCrO₂ and NaNi_1/3Mn_1/3Co_1/3O₂, etc.) and polyanions (olivine NaFePO₄, NASICON Na₃V₂(PO₄)₃, orthorhombic Na₂FePO₄F, monoclinic Na₂MnPO₄F, etc.) are being actively examined as cathode materials.¹¹ In the case of anode materials, carbonaceous materials (hard carbon, graphite, etc.), metal oxides/sulphides/selenides/phosphides (TiO₂, Na₂Ti₃O₇, Co₃O₄, MoS₂, CoSe₂, NiP₃, etc.), alloys (Si, Ge, Sn, P, Sb, etc.) and so forth have been extensively developed with varied achievements.¹² However, their performances including specific capacity, rate capability, and cycle life are severely hindered by the large ionic radius of Na (1.02 Å) and its low standard electrochemical potential (2.71 V vs. SHE) compared with those of the Li analogue (0.76 Å and 3.04 V vs. SHE), and as a result there have been extensive attempts to search for novel kinds of Na ion storage materials.

4.1 Quinone molecule/polymer/salt

Organic compounds including quinones that consist of abundant C, H, O, N elements and can be obtained from biomass resources are an attractive low-cost and sustainable choice as SIB electrode candidates.¹²⁸ What's more, organic electrodes are generally not limited by the choice of counter-ions, making them attractive for Na ion storage to overcome the large ionic radius limitation associated with Na (1.02 Å). Thirdly, quinones are significantly occupied in LIBs. The similar battery chemistry between LIBs and SIBs makes the explored LIB-based quinone electrodes adaptable to SIB applications. Li et al.¹²⁹ investigated the Na ion reversible capacity of the PhQ molecule. The PhQ molecule was encapsulated into SWCNTs to suppress its dissolution into 1 M NaClO₄ in PC electrolyte. The PhQ/SWCNT composite displayed two discharge voltage plateaus at ∼2.3 and 1.9 V vs. Na/Na⁺ with a discharge capacity of ∼200 mA h g⁻¹ at 0.1 A g⁻¹. The macrocyclic C4Q, which has previously been studied for Li⁺ storage, is also examined as a cathode candidate for SIBs.^118,130 ILs with tailorable polarity and donor properties were used as electrolyte solvents to tackle the dissolution issue of C4Q.¹¹⁸ It is revealed that ILs with weaker polarity and a lower donor number are favorable for suppressing the dissolution of quinones (Fig. 13a and b). Notably, the saturated concentration of C4Q in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([PY13][TFSI]) is 0.42 g L⁻¹ at room temperature, much lower than the 2.05 g L⁻¹ obtained in DME, which has been extensively employed as the electrolyte solvent for SIBs. Consequently, the C4Q electrode showed no evident color change in [PY13][TFSI] electrolyte for 7 days, while the DME electrolyte quickly turns yellow (Fig. 13c). Remarkably, in 0.3 M Na[TFSI]/[PY13][TFSI] electrolyte, the C4Q cathode afforded high discharge capacities of 406 mA h g⁻¹ at 0.02 A g⁻¹ and 343 mA h g⁻¹ at 0.045 A g⁻¹, along with a superb capacity retention of 99.7% at 0.13 A g⁻¹ after 300 cycles (Fig. 13d and e).¹¹⁸

The salt form of quinones has also been reported as a promising electrode candidate for SIBs. As an analogue of dilithium rhodizonate, disodium rhodizonate (Na₂C₆O₆) is actively studied as a cathode material for SIBs due to its high density of –CO groups.^131–134 If fully reduced, Na₂C₆O₆ can undergo a four-electron redox reaction to afford a theoretical capacity of 501 mA h g⁻¹. Chihara et al.¹³¹ firstly investigated the electrochemical performance of Na₂C₆O₆ as a cathode for SIBs. The Na₂C₆O₆-based electrode demonstrated a repeatable capacity of about 170 mA h g⁻¹ with an average discharge plateau of 2.18 V vs. Na/Na⁺ in 1 M NaClO₄/PC electrolyte. However, distinct capacity fading was observed when the charge cutoff potential was above 3.2 V vs. Na/Na⁺, due to the dissolution of disodiated Na₂C₆O₆ in the higher potential region. Moreover, the theoretical capacity can hardly be achieved. Later, Wang et al.¹³² examined the size-dependent sodium storage properties of Na₂C₆O₆ electrodes. Microbulk, microrod, and nanorod structured Na₂C₆O₆ were synthesized using a facile antisolvent method. The nanorod structured Na₂C₆O₆ exhibited the best performance by enabling stable contact between Na₂C₆O₆ and carbon black. A stable discharge capacity of 190 mA h g⁻¹ can be reached at 0.1C with over 90% retention after 100 cycles, which corresponds to the insertion of less than 2 Na atoms. This study signifies that rational morphological control can lead to significantly improved sodium storage performance. Recently, Lee et al.¹³⁴ studied the phase transformation of Na₂C₆O₆ during the first cycle. As illustrated in Fig. 14, during sodiation, the initial α phase of Na₂C₆O₆ is transformed into the γ phase, which is an energetically favorable process. However, during the subsequent desodiation process, the structural change from the γ phase back to the initial α phase is kinetically suppressed, which leads to premature disodiation. This irreversible phase change is the origin of the limited sodium storage during the subsequent cycles. Fortunately, the reversible structural change between α and γ phases can be achieved by reducing the particle size, selecting suitable electrolytes (diethylene glycol dimethyl ether) and controlling the charge cutoff voltage, which provides a practical strategy to realize approximately four sodium storage with a specific capacity of 484 mA h g⁻¹. This work also highlighted the importance of understanding the redox mechanism to facilitate effective utilization of active compounds.

Di-sodiated 2,5-dihydroxy-1,4-benzoquinone (Na₂C₆H₂O₄, Na₂DBQ), which is an analogue of the previously reported Li version,⁹⁸ is also employed as a sodium storage material.^135–137 Na₂DBQ contains two carbonyl groups as redox centers and can afford a high theoretical capacity of 291 mA h g⁻¹. Na₂DBQ, synthesized by Zhu et al.¹³⁵ using a simple one-pot reaction of dihydroxy-1,4-benzoquinone with sodium methylate, can operate at an average discharge voltage of ∼1.2 V vs. Na/Na⁺ with a specific capacity of 265 mA h g⁻¹ at 0.1C and a capacity retention of 76.7% over 2–300 cycles at 1C, making it a promising anode for SIBs. Wu et al.¹³⁶ introduced a Na₂DBQ/CNT composite prepared by a simple spray drying method. The hybridization with conductive networks of CNTs can offer fluent charge transport pathways. The Na₂DBQ/CNT composite with 10 wt% CNTs exhibited a storage capacity of 259 mA h g⁻¹ at 0.1C with an initial coulombic efficiency of 88% and a remaining capacity of 142 mA h g⁻¹ at 7C. Moreover, the average sodium storage voltage increased slightly to 1.4 V vs. Na/Na⁺ due to reduced polarization. Recently, Gurkan et al.¹³⁷ reported the immobilization of Na₂DBQ on high surface area ordered mesoporous carbon (OMC) and the use of IL based electrolyte (1-methyl-3-propylpyrrolidinium bis(fluoromethylsulfonyl)imide, [PYR13][FSI]) to improve the rate and cycle performance. The cell with Na₂DBQ-OMC as an active material and 1 M Na[FSI] in [P13YR][FSI] as an electrolyte exhibited capacities of 277, 226, 203, 185 and 175 mA h g⁻¹ at 60 °C at 0.025, 0.05, 0.1, 0.2 and 0.3 A g⁻¹, respectively, with a cycle life of over 300 cycles.

The combined solidification and polymerization could effectively solve the solubility issue of small quinone molecules in aprotic electrolyte. By polymerization of the above-reported Na₂DBQ with thioether bonds, an oligomeric sodium salt, namely the sodium salt of poly(2,5-dihydroxy-p-benzoquinonyl sulfide) (Na₂PDS), was successfully prepared by Wu et al. using the Phillips method.¹³⁸ In the CV profile of Na₂PDS, two pairs of redox peaks at 1.02/1.11 V and 1.38/1.44 V vs. Na/Na⁺ were observed, which overlapped into one single pair of redox peaks in the subsequent cycles, most probably due to the conjugated effect of the benzene ring and the fast reaction kinetics of carbonyl groups. The reversible capacities of the Na₂PDS electrode were 225 mA h g⁻¹ and 131 mA h g⁻¹ at 0.1 and 1 A g⁻¹. Moreover, a capacity of about 138 mA h g⁻¹ was retained after 500 cycles at 0.5 A g⁻¹.

In summary, significant progress has been made on quinone based active materials for SIB applications, including high specific capacity values (e.g., 484 mA h g⁻¹ for Na₂C₆O₆)¹³⁴ and long cycle life (over 500 cycles for Na₂PDS).¹³⁸ However, apart from the low electrical conductivity and dissolution issues, the numbers of explored quinones and their derivatives are still quite limited when compared with those of their Li analogues. Besides, as the redox potential of Na/Na⁺ (−2.71 V vs. SHE) is higher than that of Li (−3.04 V vs. SHE), the quinones should possess a lower operation potential (vs. Na/Na⁺) for SIB applications. Current reported quinones generally have a redox potential in the voltage range of 1–3 V vs. Na/Na⁺, which is slightly low for cathode application and too high for anode applications. Therefore, special attention should be paid on tuning the redox potentials of quinones in further research.

4.2 Quinone based composites

The quinone based composite also showed excellent performance in SIBs. Juglone, a renewable biomolecule derived from waste walnut epicarp, was immobilized onto rGO nanosheets via strong π–π interaction.¹³⁹ Specifically, the Juglone/rGO composite electrode was prepared by directly immersing Cu foil into the Juglone and graphene oxide (GO) solution mixture followed by hydrazine reduction. The reversible reduction and oxidation of the –CO groups was evidenced by the O K-edge X-ray absorption near edge spectroscopy (XANES) of the hybridized electrode (Fig. 15a). Theoretically, the Juglone molecule can undergo a two-electron reduction reaction to offer a specific capacity of 290 mA h g⁻¹ (Fig. 15b). The immobilization of Juglone onto rGO nanosheets not only suppressed the dissolution of the Juglone molecule, but also provided a conductive pathway for fast charge transfer, leading to dramatically improved electrochemical performance (Fig. 15c). The specific capacity of the Juglone/rGO composite stabilized at 305 mA h g⁻¹ after 10 cycles at 0.1 A g⁻¹ within 0–2.5 V vs. Na/Na⁺, and retained 212 mA h g⁻¹ after 300 cycles. Additionally, average discharge capacities of 398, 305, 250, 225, and 210 mA h g⁻¹ were achieved at 0.05, 0.1, 0.2, 0.3, and 0.4 A g⁻¹, respectively. Chen et al.¹⁴⁰ prepared an organic compound, 4,8-dihydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione (BDT), and then deposited it on graphene by a simple dispersion–deposition process. It was found that the BDT/graphene composite presented three sloping plateaus centered at ∼2.2, ∼2.1, and 2.0 V vs. Na/Na⁺ during discharging. A high initial discharge capacity of 217 mA h g⁻¹ was obtained at 0.1C, which is 89% of the theoretical capacity of the BDT material (243 mA h g⁻¹). After 70 cycles, the specific capacity of the BDT/graphene composite was still 175 mA h g⁻¹, which is much higher than the 100 mA h g⁻¹ of the BDT material.

Liu et al.¹⁴¹ coated polydopamine (PDA) on the surface of few-walled carbon nanotubes (FWNTs) through a self-polymerization process and then fabricated a free-standing and flexible hybrid electrode using a vacuum filtration method, and it was used as an organic cathode material for both LIBs and SIBs. A maximum PDA mass loading of 65.6 wt% can be achieved with a measured electrical conductivity of 20 S cm² g⁻¹. During the electrochemical performance test, the hybrid film showed multiple redox peaks in the range of 2.5–4.1 V vs. Li/Li⁺ and sloping discharge profiles due to the combination of double-layer capacitance of FWNTs and faradaic redox reactions of PDA. The gravimetric capacity of the hybrid films with 53 wt% PDA is 133 mA h g⁻¹ in Li-cells and 109 mA h g⁻¹ in Na-cells at 0.05 A g⁻¹.

5. Organic quinones for PIBs

The PIB is another promising candidate for future large-scale energy storage applications due to the high abundance and widespread availability of potassium. Moreover, the redox potential of potassium is lower than that of sodium and is comparable to that of lithium (−2.93 V vs. SHE for K/K⁺, −3.04 V vs. SHE for Li/Li⁺, and −2.71 V vs. SHE for Na/Na⁺); thus, a high energy density may be delivered for PIBs.¹⁴² Furthermore, the weaker Lewis acidity of K⁺ is favorable for the larger transfer number and higher mobility of K⁺.¹⁹ All these merits make PIBs a promising alternative to the current LIBs. Unfortunately, PIBs have not yet been widely investigated because metallic K is extremely active and has a low melting point of 64 °C, which cause serious safety concern. Moreover, most of the reported electrode materials of PIBs still suffer from limited capacity, low cycle stability and poor rate performance due to the larger ionic radius of K ions (1.38 Å) than those of Li (0.76 Å) and Na ions (1.02 Å),¹⁴³ which leads to significant structural deterioration and large volume changes of the rigid inorganic structures. Therefore, it is crucial to seek promising electrode candidates that can reversibly accommodate the bulky K ions.

Advantageously, organic electrodes including quinone materials have demonstrated significant success in both LIBs and SIBs due to their notable characteristics including high electrochemical reactivity, structural diversity, low cost and widespread abundance. In addition, compared with inorganic materials that are organized via ionic or covalent bonding, the assembly of organic materials is realized by van der Waals force, which endows organic materials with larger void spaces and a lower energy barrier for accommodating most metal ions without the size concern.¹⁴⁴ To date, encouraging results have been obtained for organic based cathodes and anodes for PIBs, such as carboxylates,^144,145 terephthalates,¹⁴⁶ dianhydrides,¹⁴⁷etc.

Quinone and its derivatives offer a new alternative to high K-ion storage materials. In a previous section, PAQS was shown to present high specific capacity and good cycling stability as a cathode for LIBs.⁸⁶ Analogously, Jian et al. investigated the K-ion storage behavior of PAQS as a cathode for PIBs.¹⁴⁸ PAQS exhibited a reversible capacity of 200 mA h g⁻¹ in potassium bis(trifluoromethane sulfonyl) imide (KTFSI)/(DOL + DME) electrolyte with a sloping voltage plateau averaged at 2.1 and 1.6 V vs. K/K⁺. Most recently, Tang et al. reported the application of poly(pentacenetetrone sulfide) (PPTS) as a cathode material for PIBs.¹⁹ This polymeric PPTS material exhibited a reversible capacity of 260 mA h g⁻¹ at 0.1 A g⁻¹ and an impressive capacity retention of >190 mA h g⁻¹ after 3000 cycles at 5 A g⁻¹. The outstanding performance is most likely attributed to the fast K-ion diffusion coefficient (up to 10⁻⁹ cm² s⁻¹).

As discussed in Section 3.3, quinone salts with –ONa and –SO₃Na polar groups showed reduced solubility in organic electrolytes and improved Li storage performance. Zhao's group explored para-disodium-2,5-dihydroxy-1,4-benzoquinone (p-Na₂C₆H₂O₆)¹⁴⁹ and the ortho-di-sodium salts of tetrahydroxyquinone (o-Na₂C₆H₂O₆)¹⁵⁰ as novel electrode materials for PIBs, and they displayed specific capacities of 190.6 mA h g⁻¹ at 0.1C (1C = 248 mA g⁻¹) and 168.1 mA h g⁻¹ at 25 mA g⁻¹, respectively. Chen's group discovered that the oxocarbon salt K₂C₆O₆ can be applied as an ultrafast K-ion insertion/extraction host with two K-ion reactions per compound (Fig. 16a–c).¹⁵¹ The K₂C₆O₆ electrode delivered a specific capacity of 212 mA h g⁻¹ at 0.2C and retained 164 mA h g⁻¹ at 10C (1C = 200 mA g⁻¹). The good performance was due to the synergistic effect of the natural semiconductor properties of K₂C₆O₆ with a narrow band gap close to 0.9 eV, the high ionic conductivity of the K-ion electrolyte, and the faster K-ion diffusion. As a proof of concept, an all organic battery with K₂C₆O₆ as the cathode and K₄C₆O₆ as the anode was assembled, and it displayed an operation voltage of 1.1 V and an energy density of 35 W hkg⁻¹ (Fig. 16d–f).

Similarly, Xu's group studied the electrochemical performance of AQDS as a cathode material for KIBs.^143,152 The polar –SO₃Na groups in the AQDS molecule can reduce the solubility of quinone in organic electrolyte and improve the K ion storage performance. As a consequence, AQDS displayed a specific capacity of 78 mA h g⁻¹ after 100 cycles at 0.1C (1C = 130 mA h g⁻¹) in 0.8 M KPF₆ in EC:DEC (1:1, v/v) electrolyte.¹⁵² Xu's group further found that a robust solid–electrolyte interface (SEI) layer could be formed on the AQDS electrode by using ether based electrolyte (i.e., 0.8 M potassium bis(fluorosulfonyl)imide (KFSI) in DME), which contributed to a significantly extended cycle life of 1000 cycles with 80% capacity retention at 3C.¹⁴³ This is because in the KFSI/DME electrolyte, the LUMO energy level of FSI⁻ is much lower than that of DME molecules (Fig. 17a), indicating that the KFSI will decompose prior to the DME solvent. During the 1^st cycle, the reduction products of the KFSI formed a dense inorganic inner layer on the surface of the AQDS electrode, which can effectively mitigate the subsequent decomposition of DME (Fig. 17b). In the EC/DEC electrolyte, the LUMO energy levels of FSI⁻, EC, and DEC species are very close (Fig. 17a), suggesting their simultaneous decomposition. The resulting mosaic structured SEI film is more likely to fracture during cycling, leading to continuous decomposition of the EC/DEC electrolyte, and an inferior cyclability (Fig. 17c).

Vitamin K, a 2-methyl-1,4-naphthoquinone derivative, was applied as an anode material for PIBs.⁴⁶ This biomolecule can reversibly store and release two potassium ions per formula unit with a theoretical capacity of 313.5 mA h g⁻¹. Graphene nanotubes (GNTs) with a large surface area and high electrical conductivity were mixed with Vitamin K (VK) to prepare a VK@GNT composite, which achieved a high reversible capacity of 300 mA h g⁻¹ and retained 222.3 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹. When tested at higher current densities of 200, 500, and 1000 mA g⁻¹, the composite retained high capacities of 203, 181, and 165 mA h g⁻¹, respectively.

Though organic quinone electrodes provide an appealing opportunity for highly reversible K-ion storage, the number of explored quinones and their derivatives is still quite limited when compared with that of their Li and Na analogues. It is envisioned that more Li and Na analogues will be explored as high capacity cathode and anode candidates for PIBs in the near future.

6. Organic quinones for MIBs

In comparison with Li, a secondary battery with a Mg metal anode exhibits attractive features such as widespread availability of Mg resources in the Earth's crust, higher volumetric capacity (3833 mA h cm⁻³ for Mg vs. 2046 mA h cm⁻³ for Li), and improved safety properties due to reduced reactivity towards air and moisture.¹⁵³ More appealingly, the reduction potential of Mg is only 0.67 V higher than that of Li (−2.37 V vs. SHE for Mg/Mg²⁺ and −3.04 V vs. SHE for Li/Li⁺); therefore, the energy density of Mg-based batteries will not be seriously compromised. Besides, the plating and stripping of Mg has a lower propensity toward dendrite formation.¹⁵⁴ Despite this low tendency, the formation of Mg dendrites is still inevitable and was reported in a few literature studies.^155,156 For example, Davidson et al. demonstrated the electrochemical growth of fractal Mg dendrites from Grignard reagents in symmetric Mg–Mg cells under galvanostatic conditions.¹⁵⁴

Inspired by the pioneering work of Aurbach et al. in 2000 with the design of the prototype Mg–Mo₆S₈ battery,¹⁵⁷ research interest in MIBs bloomed. On the one hand, intensive research is going on in the field of electrolytes that are capable of plating/stripping Mg reversibly with wide electrochemical windows. On the other hand, a large number of attempts were made to explore electrode materials that are capable of storing Mg²⁺ ions reversibly. To date, only a few types of inorganic materials, such as Mo₆S₈,¹⁵⁷ MnO₂,¹⁵⁸ H₂V₃O₈,¹⁵⁹ and water activated VOPO₄,¹⁶⁰ have been reported to facilitate Mg²⁺ ion intercalation. This is presumably because the divalent Mg²⁺ interacts more strongly with the rigid lattice than monovalent cations and hardly diffuses in inorganic crystals.¹⁶¹ As discussed in PIBs, the quinone materials, which are organized by weak van der Waals force, pose only a modest Coulomb repulsion to the diffusing cations. Furthermore, their malleable and soft lattice may allow molecular reorientation for facile and reversible intercalation of hard divalent cations.¹⁶² These properties endow quinone materials with a lower energy barrier for accommodating Mg²⁺. Therefore, redox-active quinone materials represent another type of promising alternative for MIBs.

Sano et al. firstly investigated the electrochemical properties of DMBQ in Mg(ClO₄)₂/γ-butyrolactone electrolyte.¹⁶¹ The DMBQ electrode demonstrated a discharge capacity of 260 mA h g⁻¹ (based on the mass of DMBQ) with two voltage plateau regions at 1.1 V and 0.8 V vs. Mg/Mg²⁺, respectively. Unfortunately, DMBQ can only be cycled several times. Later, Pan et al. found that magnesium bis(trifluoromethylsulfonyl)imide mixed with MgCl₂ in DME (Mg(TFSI)₂–2MgCl₂ in DME) electrolyte endowed the DMBQ electrode with a high discharge potential above 2.0 V vs. Mg/Mg²⁺.¹⁶³ This signifies that the use of organic quinones in combination with carefully chosen organic electrolytes is a very promising strategy to achieve high battery performance.

Anthraquinone-based polymers were also explored as cathode candidates for MIBs. PAQS, which is a well-known polymer electrode for LIBs, was examined for MIBs by Bitenc et al.¹⁶⁴ Specific capacities between 150 and 200 mA h g⁻¹ at a voltage of 1.5–2.0 V vs. Mg/Mg²⁺ were obtained. Later, Pan et al. reported improved electrochemical performance with two new anthraquinone-based polymers, namely 2,6-polyanthraquinone (26PAQ) and 1,4-polyanthraquinone (14PAQ).⁴⁷ The CV profiles of 26PAQ at 0.5 mV s⁻¹ exhibited two reduction peaks at 1.71/1.52 V vs. Mg/Mg²⁺ and one oxidation peak at about 2.0 V vs. Mg/Mg²⁺, corresponding to the stepwise reaction between neutral 26PAQ and [26PAQ]²⁻ anions (Fig. 18a). Similarly, 14PAQ displayed two reduction peaks at 1.57/1.48 V vs. Mg/Mg²⁺ and two oxidation peaks at 1.67/1.79 V vs. Mg/Mg²⁺, respectively (Fig. 18b). Note that the redox potential of 26PAQ was slightly higher than that of 14PAQ due to different structural configurations. In the subsequent charge/discharge test, 26PAQ delivered an initial discharge capacity of 122 mA h g⁻¹ at 0.5C (130 mA g⁻¹) and retained 100.2 mA h g⁻¹ after 100 cycles, corresponding to a capacity retention of 82% (Fig. 18c and e). In contrast, considerable capacity loss from 132.7 to 106.0 mA h g⁻¹ was observed for 14 PAQ in the first seven cycles. However, very little capacity loss was detected in the subsequent cycles. The remaining capacity for 14PAQ was 104.9 mA h g⁻¹ after 100 cycles (Fig. 18d and f).

In order to circumvent the sluggish Mg²⁺ transport in host lattices, a Mg–Li dual-salt electrolyte has recently been proposed,¹⁶⁵ and it is characterized by Li-intercalation or Mg–Li co-intercalation at the cathode and Mg plating/stripping at the anode. Tian et al. reported a Mg–Na₂C₆O₆ system activated by using Mg–Li dual-salt electrolyte.¹⁶⁶ The nanocrystalline rhodizonate salt Na₂C₆O₆ (denoted as n-SR) with rGO as a conductive additive demonstrated high specific capacity and excellent rate performance. The obtained reversible discharge capacities were 450, 300, 250, 200 and 175 mA h g⁻¹ at 0.05, 0.5, 1, 2.5 and 5 A g⁻¹, respectively (Fig. 19a). The corresponding discharge–charge curves showed capacitor-like characteristics (Fig. 19b), which coincided with the CV analysis. Such an extraordinary performance endowed the Mg–Na₂C₆O₆ system with a high energy density of 525 W h kg⁻¹ at a specific power of 125 W kg⁻¹ (based on the active cathode material), and these results greatly surpassed those of the typical high-voltage inorganic structures (Fig. 19c). They also elucidated that the redox mechanism of Na₂C₆O₆ was dominantly driven by Li⁺. As illustrated in Fig. 19d, the first discharge process enabled a four-electron reaction from Na₂C₆O₆ to Na₂Li₄C₆O₆, which was however oxidized to Na₂LiC₆O₆ (rather than Na₂C₆O₆). In the subsequent cycles, the reversible reaction roughly occurred between Na₂Li₄C₆O₆ and Na₂LiC₆O₆.

Although great success has been achieved for organic quinone cathodes in LIBs and SIBs, only a handful of quinone cathodes have been reported for MIBs in the literature, which is the same status as that of PIBs. Exploration of novel quinone structures with high-voltage and large Mg ion storage capacity and in-depth understanding of their reaction mechanism are greatly needed in future.

7. Organic quinones for aqueous ZIBs

Apart from nonaqueous Li/Na/K/Mg ion energy storage systems, aqueous rechargeable batteries have attracted increasing interest, and they not only offer potential cost benefits but also favor high rate capabilities due to the high ionic conductivities of aqueous electrolytes. Among them, aqueous ZIBs are particularly intriguing owing to the unique properties of Zn, which include high theoretical capacity (820 mA h g⁻¹), low redox potential (−0.763 V vs. SHE), high stability in aqueous solution, high natural abundance and low toxicity.^124,167 More importantly, Zn can be easily stripped/plated from aqueous media, which is favorable for aqueous energy storage systems.

Until now, considerable efforts have been made in exploring inorganic compounds for ZIBs, including manganese (Mn)-based oxides, vanadium(V)-based oxides and Prussian blue analog-based materials.^168,169 Unfortunately, these inorganic compounds are more likely to involve toxic and/or environmentally unfriendly elements when used as cathode materials for ZIBs. For example, vanadium compounds are highly toxic and Prussian blue analogs will decompose to form highly toxic cyanide CN⁻ under acidic conditions (most of the electrolytes for ZIBs are slightly acidic). Very recently, organic electrode materials have been investigated as low-toxicity and sustainable alternatives to conventional inorganic cathode materials for ZIB applications. The versatile properties of quinones, including high theoretical capacity, structural diversity, high abundance, low cost, and green sustainability, offer an interesting alternative to rigid inorganic structures. Quinone compounds store charge via an ion-coordination mechanism where the cations coordinate to the negatively charged oxygen atoms upon electrochemical reduction of the carbonyl groups, and uncoordinate reversibly during the reverse oxidation.¹⁵ In other words, the cations act to compensate for the charge of quinone-based electrochemical reactions. This makes the storage of hard divalent Zn²⁺ highly reversible.

In 2017, Chen's group first explored quinone compounds as cathode materials for ZIBs.²⁰ They tested a series of quinone compounds with carbonyls in the para-position and found that C4Q with open bowl structures and eight carbonyls exhibited a high capacity of 335 mA h g⁻¹ and a flat operation voltage of 1.0 V vs. Zn/Zn²⁺ with a small voltage polarization of 70 mV. Different to inorganic materials, organic materials usually possess less crystallinity, making it difficult to accurately characterize them with ex situ/in situ approaches such as XRD and TEM. For this reason, Chen's group utilized the electrostatic potential (ESP) method, in situ attenuated total reflection-Fourier transform IR (ATR-FTIR) spectroscopy, Raman spectroscopy, and ultraviolet-visible (UV-vis) spectroscopy to investigate the structural evolution and dissolution behavior of quinone compounds during discharge and charge processes. It is known that electrophilic reactions tend to occur in the reactive sites with more negative ESP. In other words, the sites with more negative ESP are more favorable for discharge reactions since they involve Zn²⁺ uptake. For C4Q, the carbonyl groups are the reactive sites since they possess lower ESP than bilateral carbonyls, revealing that they have strong interactions with Zn²⁺ ions. During the discharge process, two carbonyl groups of C4Q will accept electrons as well as storing Zn²⁺ ions. Upon charging, C4Q will be oxidized and hence the captured electrons and Zn²⁺ ions will be released. The redox reaction of Zn²⁺ uptake in C4Q can be expressed as follows:

C4Q + Zn2+ ↔ Zn3C4Q	(1)

However, due to the soluble characteristics of the discharge products of C4Q, an expensive cation-exchange membrane (Nafion) was employed as a separator to suppress the cross-over of C4Q^2x− and prevent the zinc anode from being poisoned by the discharge products.

Recently, a small quinone molecule, tetrachloro-1,4-benzoquinone (p-chloranil), was explored as a novel organic cathode material for ZIBs.¹⁶² It offers a high capacity of 205 mA h g⁻¹ with a very small voltage polarization of 50 mV at a flat plateau at around 1.1 V vs. Zn/Zn²⁺ (Fig. 20a and b). Through FTIR, EDX, and EIS studies, a unique phase transformation between p-chloranil and Zn-p-chloranil via a water assisted phase transfer mechanism was revealed (Fig. 20c). During the discharge process, p-chloranil reacts with H₂O protons to form quinols at the solid/electrolyte interface, and then quinols react with Zn²⁺ ions in the electrolytes to form insoluble Zn-p-chloranil. Upon charging, Zn-p-chloranil exchanges protons with H₂O to form soluble quinols, which then oxidize at the carbon surface and grow as rhombus particles of p-chloranil. DFT calculations further unraveled the structural evolution details of p-chloranil, in which the molecular columns undergo a twisted rotation to accommodate Zn²⁺, thus alleviating the volume change during cycling.

The main issue for the application of quinone compounds in aqueous ZIBs is the continuous dissolution of quinone compounds or the discharged quinone products in aqueous electrolytes, which would result in a rapid capacity fading. Similar to the strategies in LIBs, one effective approach to alleviate this dissolution issue in aqueous electrolytes is the polymerization of small organic carbonyl compounds. Chen's group investigated PBQS as a novel polymer cathode for ZIBs.¹⁷⁰Ex situ infrared (IR) spectral data and theoretical calculations revealed that CO functional groups in PBQS are active reaction sites and the energy storage mechanism of PBQS is ascribed to the reversible bonding of Zn²⁺ ions with carbonyl oxygen atoms (Fig. 20d and e). The as-prepared PBQS delivered a high capacity of 203 mA h g⁻¹ at 0.1C with an average operation voltage of 0.95 V vs. Zn/Zn²⁺ and a high capability of 126 mA h g⁻¹ at 5.0C (Fig. 20f).

These reports on quinone compounds will facilitate the development of organic materials as sustainable and environmentally friendly cathodes for aqueous ZIBs. Although quinone compounds show great potential in aqueous energy storage systems, their development is still in its infancy. Exploration of other high-voltage and large-capacity quinone electrodes for ZIBs and in-depth understanding of their reaction mechanism are greatly needed in future.

8. Organic quinones for SCs

Electric double layer supercapacitors (EDLCs), which store charge through ion adsorption to form electric double layers at the surface between the EDLC electrode and electrolyte, are considered as a novel energy storage device with superior cycle life and high power density. The most intensively utilized electrode materials for EDLCs are various carbons with a porous structure, high specific surface area (SSA) and/or abundant heteroatom doping.¹³ Nonetheless, due to the restricted charge accumulation at the activated carbon (AC) electrode, EDLCs deliver inferior capacitance, which will restrict the energy density of EDLC devices to <10 W h kg⁻¹. Although the reasonable and innovative design of carbon based materials could enhance the energy density, it still can't satisfy the requirements for portable electronics, electric vehicles and smart grid stations.

In this regard, pseudocapacitive materials, which include metal oxides (e.g., Co₃O₄, Fe₂O₃, etc.), metal sulfides (e.g., MoS₂, CoNiS₄, etc.), metal hydroxides (e.g., NiOH, Co(OH)₂, etc.), conducting polymers (e.g., polypyrrole, polyaniline, etc.) and redox-active organic molecules, have been extensively investigated due to their large specific capacitance, superior energy density, and high power density along with reasonable cycling stability.^13,14 Among the various redox-active organic molecules, quinones have shown great potential for pseudocapacitors owing to their considerably high theoretical capacity, feasible structural design flexibility, superior electrochemical reversibility, low cost and environmental friendliness. Thus, a number of quinone molecules have been studied, including 9,10-AQ,¹⁷¹ anthraquinone-2,6-disulfonic acid disodium salt,¹⁷² 1,4,5,8-tetrahydroxy anthraquinone (THAQ),¹⁷³ 1,8-dihydroxyanthraquinone (DT),¹⁷⁴ DMBQ,¹⁷⁵etc.

In spite of their unique features, the electrochemical properties of quinone based electrodes are subjected to the relatively low intrinsic electrical conductivity and high solubility in electrolyte. As a result, extensive efforts have been devoted to solving the above issues, and introducing a conductive scaffold (carbonaceous material, conducting polymer, etc.) has been found to be an effective strategy to obtain high performance quinone electrodes for pseudocapacitors. The conductive scaffold can not only ensure rapid electron transport, but can also guarantee an improved cycling stability by suppressing the dissolution of quinones.

8.1 Quinone/porous carbon composites

Porous carbon, containing abundant micropores and/or mesopores and/or macropores, has been extensively employed as a conductive substrate to integrate with quinones due to its ultrahigh SSA, considerable EDLC capacitance, easy processibility and low cost. Previously, 1,4,9,10-anthracenetetraone (AT), which can exchange up to four electrons, has been studied as a redox active material and was modified with two different carbon electrodes (Vulcan XC-72R, i.e., Vulcan, and Picactif BP 10, i.e., Pica).¹⁷⁶ It is found that the formation of covalent bonds between AC and quinones is an effective approach to enhance the performance of quinones/AC composites. Such composite electrodes (i.e., Vulcan/AT and Pica/AT) demonstrated improved total capacitances as well as an enhanced capacitance retention of 90% for Pica/AT and 60% for Vulcan/AT. Afterwards, Belanger and co-authors studied two entirely different methods to modify Black Pearls (BP) carbon with 9,10-PQ, i.e., grafting by reduction of the corresponding in situ generated diazonium cations (PQ-grafted BP), or soaking in PQ/acetonitrile solution (PQ-adsorbed BP).¹⁷⁷ As a result, PQ-adsorbed BP suffered from a relatively poor long-term stability compared to PQ-grafted BP which has strong covalent bonds between the PQ molecules and BP carbon matrix. Moreover, the stability of PQ-grafted BP was also superior to that of the AQ-grafted BP, which may be attributed to the proximity of the ketone features of PQ molecules.

Additionally, Won et al. synthesized a quinone containing conductive additive, 2,5-bis((2-(1H-indol-3-yl)ethyl)amino)cyclohexa-2,5-diene-1,4-dione (HBU), via a chemical method and then mixed it with AC to yield a composite electrode,¹⁷⁸ and this provides a new avenue to develop excellent quinone based electrodes through stem grafting various quinone molecules together. Consequently, SCs adopting the composite electrode delivered specific capacitance up to 130 F g⁻¹ at various scan rates ranging from 100 to 1000 mV s⁻¹. In Lei's research, three different quinones – AQ, amino-anthraquinone (1-AAQ), and 2-amino anthraquinone (2-AAQ) – served as redox active materials to decorate carrot derived carbon skeletons (CCs) via physical adsorption.¹⁷⁹ The CC modified with 1-AAQ showed the best capacitive behavior. It delivered a high specific capacitance of 328 F g⁻¹ at 0.5 A g⁻¹ and excellent cycling stability of 95% after 5000 cycles at 3 A g⁻¹. Regrettably, the author did not delve into the mechanism of the performance differences between the three quinones.

Apart from AC, ordered mesoporous carbons (OMCs) are also some of the most novel carbonaceous materials with improved electrical conductivity. Therefore, Yan and co-authors developed an AQ-modified OMC (AQ/OMC) via a solvothermal method,¹⁷¹ and it not only possesses mesoporous channels to promote fast ion diffusion, but also contributed additional pseudocapacitance to greatly enhance the specific capacitance. The AQ/OMC demonstrated specific capacitance as high as 346 F g⁻¹ at 0.5 A g⁻¹, together with an excellent capacitance retention of 84.3% under a high current density of 30 A g⁻¹ due to the synergic effect of AQ and OMC.

8.2 Quinone/graphene composites

Compared with porous carbon, graphene has a higher electrical conductivity, extremely outstanding compatibility with various components and abundant surface chemistry, which make it a promising conductive substrate. Previously, Shi's group reported a 1-AAQ modified self-assembled graphene hydrogel (AQSGH),¹⁸⁰ which was synthesized through covalently grafting 2-AAQ on chemically modified graphene (CMG) sheets. As a result, the capacitance of AQSGH was improved to 258 F g⁻¹ at 0.3 A g⁻¹, which is much larger than that (193 F g⁻¹) of pure CMG. This is mainly due to the covalently bonded AAQ moieties contributing additional redox capacitance. In addition, in Shi's opinion, a high-performance supercapacitor electrode with quinone molecules should be satisfactory if it meets the following requirements: excellent conductivity, strong interaction between the quinones and conductive scaffold, high density loading of quinones, and a substantial number of pores and considerable SSA of the conductive scaffold. Benefiting from the conjugated structure and the amine groups (–NH₂) of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (ANS), the ANS can interact with graphene via π–π interactions and covalent bonding simultaneously.¹⁸¹ In the ANS modified reduced graphene oxide (ANS-rGO) composite, the rGO showed an excellent graphitization degree, and the hydroxyl and sulfonic acid groups can greatly increase the dispersion stability of the composite. Therefore, ANS-rGO displayed a superior specific capacitance of 375 F g⁻¹ at 0.3 A g⁻¹, and excellent cycling stability (97.5% capacitance retention after 1000 cycles).

However, covalent functionalization of graphene inevitably disrupts the conjugated structure of graphene by conversion of carbon atoms from sp² to sp³ hybridization, which leads to decreased electrical conductivity of graphene. In contrast, non-covalent (e.g., π–π stacking interactions, van der Waals force, etc.) decoration does not significantly destroy the π-conjugated system of graphene, indicating that the non-covalent functionalization is an effective approach to inhibit the dissolution of quinones while retaining the high conductivity of graphene. Therefore, An et al. prepared an anthraquinone derivative, alizarin (AZ) immobilized graphene hydrogels (SGHs), through completely π–π interaction,¹⁸² and it was used as a promising pseudocapacitive material for SCs and demonstrated outstanding performance. Analogously, the DT molecule, providing a fast and reversible 4e⁻/4H⁺ redox reaction, has been utilized as a novel redox active material to decorate rGO nanosheets.¹⁷⁴ As a result, the DT–rGO composite exhibited an excellent capacitance up to 491 F g⁻¹ at 1 A g⁻¹, along with a superior electrochemical stability of 98.8% after 10000 cycles, outperforming a large number of previously reported quinone based electrodes. Comparatively, Gogotsi and co-authors developed DMBQ modified rGO (DMBQ@rGO) via a simple hydrothermal method.¹⁷⁵ The as-prepared DMBQ@rGO composite delivered an ultrahigh capacitance up to 650 F g⁻¹ at 5 mV s⁻¹, outperforming a large number of previously reported electrodes. Importantly, they also investigated various mass ratios of DMBQ:rGO and the results show that the capacitance of the composite increased and then decreased. Thus, moderate loading of quinones can effectively enhance the electrochemical performance.

In our previously study, THAQ with abundant carbonyl groups was firstly explored as a novel redox active electrode material for SCs and a THAQ/rGO composite was obtained by a hydrothermal growth method.¹⁷³ Different from the reported AQ, DMQ, ANS molecules, etc., the THAQ in this composite was grown into one dimensional nanorods, which were uniformly and strongly anchored into the rGO network. The optimized THAQ/rGO composite demonstrated a relatively superior capacitance of 259.0 F g⁻¹ and a high capacitance retention of 97.9% after 10000 cycles. Furthermore, the THAQ/rGO was filtered into filter paper (FP) to obtain a flexible electrode (THAQ/rGO@FP) (Fig. 21a), which was then assembled into an all-solid-state SC with high capacitive performance and rigorous mechanical properties. However, the recrystallization of THAQ resulted in a limited hybridization degree between THAQ and rGO. To achieve more uniform hybridization, AQS molecules were employed as a quinone based organic material due to their superior hydrophilicity enabled by the presence of the –SO₃⁻ functional group, which not only provides a molecular-level hybridization of AQS and rGO, but also serves as a molecular spacer to prevent the aggregation of rGO sheets.²³ On the above basis, a 3D aerogel AQS@rGO composite was successfully prepared (Fig. 21b), and it delivered an ultrahigh specific capacitance of 567.1 F g⁻¹ at 1.0 A g⁻¹ with a capacity retention of 89.1% after 10000 cycles at 10.0 A g⁻¹ (Fig. 21c and d). More importantly, DFT calculation revealed that AQS offers strong adhesion to rGO sheets with the formation of a space-charge layer (Fig. 21e and f), which was favorable for the charge transfer from graphene to the pseudocapacitive AQS. This work provides a new avenue for developing high performance SCs via rational combination of redox organic molecules with highly conductive graphene.

8.3 Quinone/CNT or CNF composites

Apart from porous carbon and graphene substrates, CNTs and CNFs can also be utilized as conductive frameworks to load the quinone compounds, although relatively little research has been done in this area. Guo et al. prepared hierarchical porous carbon nanotubes (HPCNTs) by carbonization and activation of polypyrrole (Ppy) nanotubes.¹⁸³ These HPCNTs exhibited abundant micropores and mesopores with an ultrahigh SSA of 2080 m² g⁻¹. Then, AQ molecules were adsorbed onto the surface of the HPCNTs with π–π stacking interactions to prepare an AQ-HPCNT composite, which showed a remarkable capacitance increment in comparison with pure HPCNTs. The AQ-HPCNTs with an optimized mass ratio of 7:5 delivered a high specific capacitance of 710 F g⁻¹ at 1 A g⁻¹.

Recently, biomass derived materials have attracted increasing attention due to their natural abundance and low cost. Wang and co-authors prepared a cheap but high capacitive performance carbon fiber electrode from natural cotton materials, in which molten sodium metal was employed to activate natural cotton for obtaining hierarchical porous graphitic carbon fibers (HPGCFs) (Fig. 22a).¹⁸⁴ To further increase the specific capacitance of HPGCFs, the AQ molecules was selected to modify the HPGCFs through non-covalent π–π stacking interactions to obtain the AQ-HPGCF composite (Fig. 22a). The CV profiles of the AQ-HPGCF composite showed well-defined and reversible redox peaks at −0.13 V and −0.16 V vs. SCE on the top of the EDLC, which is ascribed to the two-electron and two-proton process of AQ molecules (Fig. 22b and c). The as-prepared AQ-HPGCFs achieved a maximum specific capacitance of 347 F g⁻¹ at a low current density of 2 A g⁻¹, and could still retain 198 F g⁻¹ (about 57% retention) even at a current density of 60 A g⁻¹ (Fig. 22d). Furthermore, asymmetric SCs based on the AQ-HPGCF negative electrode and HPGCF positive electrode have been assembled and they delivered a maximum energy density of 19.3 W h kg⁻¹.

8.4 Quinone/other carbon composites

Onion-like carbons (OLCs), consisting of abundant spherical carbon nanoparticles, have been investigated as a promising material for high power-handling applications due to the absence of intraparticle porosity and the dominance of external surface area. Yet, the absence of intraparticle porosity and limited SSA lead to relatively low specific capacitance. Currently, introducing redox active quinone materials is one of the most effective ways to significantly enhance the electrochemical performance. Presser's group investigated OLC electrodes decorated with 1,4-NQ, 9,10-PQ and 4,5-pyrenedione (PY),¹⁸⁵ and the maximum capacitance of 264 F g⁻¹ was found for the PY decorated OLC (PY-OLC), which also displayed the highest capacitance retention (almost 3% fade in specific capacitance) after 10000 cycles. In a follow-up study, Presser's group reported a new synthesis method to obtain a free-standing, highly conductive electrode consisting of OLC and carbonized polyacrylonitrile (PAN) fibers (CFs).¹⁸⁶ The as-prepared OLC/CF composite with superior rate performance served as a conductive substrate for facile quinone decoration. The decoration of the composite electrode with PQ quinone increased the capacitance more than eight times to 288 F g⁻¹ in 1 M H₂SO₄ electrolyte with stable cycling for 10000 cycles.

Additionally, conducting polymers (including Ppy and polyaniline (PANI)), which demonstrate high conductivity, high theoretical capacitance, easy polymerization, and good stability in an ambient atmosphere, are a kind of promising pseudocapacitive electrode material for SCs. They can also be utilized as a conductive matrix to support quinone molecules. In an early study, Zhang et al. synthesized a PANI film and further obtained an AQS/PANI composite through a novel electrochemical doping–dedoping–redoping method on a pre-activated pure graphite electrode surface.¹⁸⁷ Unfortunately, this AQS/PANI composite served as a catalyst for the oxygen reduction reaction. Afterward, Han et al. reported an AQS and anthraquinone-2,6-disulfonic acid disodium salt (AQDS) modified graphene/polypyrrole nanocomposite (GPy) (AQ(D)S-GPy) for high performance SCs.¹⁷² Firstly, the AQ(D)S was adsorbed on the basal plane of GO through non-covalent stacking to prepare AQ(D)S modified GO, which then served as an oxidizing active agent to in situ polymerize pyrrole. The physical characterization confirmed the successful polymerization of polypyrrole and the reduction of GO. Additionally, the AQ(D)S in the AQ(D)S-GPy composite also served as a redox modifier to provide considerable pseudocapacitance to significantly enhance the electrochemical properties. As expected, the as-developed AQ(D)S-GPy composite provided outstanding specific capacitances of 237 and 300 F g⁻¹, respectively, as well as excellent cycling stability. Moreover, symmetric SCs based on AQDS-GPy exhibited a high energy density of 31.2 W h kg⁻¹ at 1.12 kW kg⁻¹ and a cycling stability of 86% after 2000 cycles.

9. Organic quinones for RFBs

Compared with solid-electrode based metal ion batteries and SCs, the operating principle of RFBs is significantly different. As shown in Fig. 23, dissolved redox-active materials are stored in the external electrolyte reservoirs and are pumped through the porous electrode. The porous electrode serves to provide active sites for charge transfer without participating in electrochemical reactions.¹⁸⁸ In this way, the stored energy and power are decoupled, with the former determined by the amount of electrolyte and the latter by the size of the cell and the number of cell stacks. This feature enables high scalability and independent control over energy and power, making RFBs an important medium and large-scale energy storage system.

The majority of RFBs reported to date are based on inorganic redox-active materials with an emphasis on metal-based species, and the redox reactions rely on the valence change of the metal centers. Common varieties include all-vanadium redox batteries (VRBs), zinc//bromine (Zn//Br₂), iron//chromium (Fe//Cr), zinc//cerium (Zn//Ce), zinc/polyiodide, all iron, all copper, hydrogen//bromine (H₂//Br₂), lithium//iodine, etc.²⁴ Similar to metal-ion batteries and SCs, the widespread implementation of inorganic RFBs is hindered by the high cost, material scarcity, environmental concerns, and uncompetitive performance metrics of metal-based species. Recently, the application of quinone compounds has also been extended to RFBs due to their potentially low cost, scalability, and efficient biodegradability. Moreover, the highly tailorable chemical and physical properties of quinone materials provide an excellent opportunity to construct sustainable and green energy-storage devices. In stark contrast to decreasing the solubility of quinone species in metal-ion batteries and SCs, the development of RFBs pursues an opposite direction in terms of high solubility, since the energy density of an RFB is proportional to the concentration of redox species. Several excellent reviews have provided comprehensive assessments of quinone compounds for RFB applications.^24,60,188 Therefore, here we will briefly elaborate on the exploitation of quinone molecules in RFBs.

The first quinone-based RFB was demonstrated in 2009 by Xu et al., and it was a single flow acid Cd–chloranil battery.¹⁸⁹ The negative electrode was cadmium and the positive electrode was an insoluble organic material, tetrachloro-p-benzoquinone (chloranil). An aqueous intermixture of H₂SO₄–(NH₄)₂SO₄–CdSO₄ was used as the supporting electrolyte. During the charging process, tetrachloro-p-benzo-hydroquinone is oxidized to chloranil at the positive electrode and cadmium ions are reduced to cadmium and electroplated onto the Cu current collector at the negative electrode. The single flow acid Cd–chloranil cell achieved an average coulombic efficiency of 99% and an energy efficiency of 82% over 100 cycles at a current density of 10 mA cm⁻². Shortly thereafter, RFBs with organic species were greatly developed.

A series of quinone derivatives were developed with tailored chemical and physical properties. Yang et al. demonstrated an aqueous organic redox flow battery with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) at the positive electrode and anthraquinone-2,6-disulfonic acid (AQDS) at the negative electrode.¹⁹⁰ During the initial cycles of charge and discharge, BQDS was converted to 1,2,4,6-tetrahydroxybenzene-3,5-sulfonic acid via two steps of electrochemical oxidation and two steps of water addition through the Michael reaction. A 25 cm² flow cell with 1 M AQDS/BQDS could be cycled at 100 mA cm⁻² with 100% coulombic efficiency over 100 cycles. Mulcahy et al. added AQDS into the electrolyte of a vanadium redox flow battery.¹⁹¹ The addition of AQDS was found to increase the capacity efficiency by an average of 7.6% over that of the control VRFB containing no AQDS. Unfortunately, the overall cycle duration was reduced by 18%. In 2014, Huskinson et al. reported a quinone-bromide RFB with AQDS on the negative side and Br⁻/Br₂ on the positive side, and it was operated at pH = 0 with a practical E_cell = 0.86 V.^192,193 Recently, Khataee et al. reported that the redox potential of many organic redox species has a strong dependence on pH. In this regard, they demonstrated a pH differential quinone-bromide RFB, which used bromine at pH ∼ 2 on the positive side and anthraquinone-2,7-disulfonate disodium (Na₂AQDS) operated at pH ∼ 8 on the negative side.^194,195 This increased pH differential RFB showed an increased E_cell of 1.3 V and was remarkably stable for at least 14 days (∼100 cycles).¹⁹⁴

A number of approaches have been pursued to enhance the solubility and reaction kinetics of quinone species. Sun et al.¹⁹⁶ presented two approaches including molecular structure engineering and utilizing an inexpensive catalyst to enhance the electrochemical kinetics of 2,5-dihydroxy-3,6-dimethyl-1,4-benzoquinone (DMBQ). They first replaced the two methyl groups of DMBQ with two methoxy groups and synthesized 2,5-dimethoxy-3,6-dihydroxy-1,4-benzoquinone (DMOBQ). The DMOBQ showed slightly higher redox potential and a narrowed gap between oxidation and reduction peaks. CV measurements of DMOBQ under the same conditions yielded an oxidation–reduction peak separation of 260 mV, which is much smaller than that (440 mV) of DMBQ (Fig. 24a and b). The second approach is to catalyze the electron transfer reaction of DMBQ with an inexpensive catalyst, namely nitrogen and sulfur codoped porous carbon microrods (N/SCMRs), which could significantly narrow the oxidation–reduction peak separation to a small value of 80 mV (Fig. 24c). As a consequence, by exploiting these strategies, improved redox kinetics could be observed, which include higher power capability of the corresponding cell and higher energy efficiency during long-term cell cycling. The K₄Fe(CN)₆/DMBQ cell assembled with bare carbon paper electrodes showed a peak power density of 113.6 mW cm⁻² at a current density of 204.5 mA cm⁻² and yielded an area specific resistance (R_s) and an electron transfer resistance (R_ct) of 1.73 and 0.52 Ω cm², respectively. In contrast, the K₄Fe(CN)₆/DMOBQ cell assembled with bare carbon paper electrodes exhibited a peak power density of 169.7 mW cm⁻² at 267.1 mA cm⁻², along with R_s and R_ct of 1.68 and 0.27 Ω cm², respectively. Furthermore, the K₄Fe(CN)₆/DMBQ cell assembled with N/S-CMR coated carbon paper electrodes showed a peak power density of 182.6 mW cm⁻² at 281.9 mA cm⁻², while the R_s and R_ct of the cell are only 1.41 and 0.23 Ω cm², respectively.

Another step forward involves utilizing an alkaline electrolyte instead of the conventional sulfuric acid electrolyte. In this way, more quinone derivatives can be considered while rationally screening redox species. In 2015, Aziz's group demonstrated a 2,6-dihydroxyanthraquinone (2,6-DHAQ)/ferrocyanide based alkaline RFB.¹⁹⁷ The 2,6-DHAQ with electron-donating OH groups has lower reduction potential. In an alkaline electrolyte, these hydroxyl groups are deprotonated to provide a high room-temperature solubility of >0.6 M in 1 M KOH. Moreover, the redox potential of quinones further shifts to more negative values in alkaline electrolyte. The cell with 0.5 M 2,6-DHAQ as the negative electrolyte and 0.4 M K₄Fe(CN)₆ as the positive electrolyte reached an open circuit voltage of 1.2 V at 50% state of charge and could be cycled at a constant current density of 0.1 A cm⁻² for 100 cycles with current efficiency exceeding 99% and a round-trip energy efficiency of 84%. Most recently, Aziz's group functionalized 2,6-DHAQ with highly alkali-soluble carboxylate terminal groups.¹⁹⁸ The resulting negative electrolyte material 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy)dibutyrate (2,6-DBEAQ) was six times more soluble than 2,6-DHAQ at pH 12. Pairing 2,6-DBEAQ with a ferro-/ferricyanide-based positive electrolyte resulted in a battery with an open-circuit voltage of 1.05 V and a peak galvanic power density of 0.24 W cm⁻² at 100% SOC. In addition, a small capacity fade rate of 0.05%/day or 0.001%/cycle was observed over a 5 day test period.

The potential range of an aqueous flow system is generally restricted by the electrolysis of water. In contrast, a non-aqueous flow battery can avoid these constraints by taking advantage of the high solubility of quinones in aprotic electrolyte with a wide potential window. To this end, Ding et al. systematically studied the electrochemical characteristics of five quinone molecules, which are 1,4-BQ, 1,4-NQ, 9,10-PQ, AQ, and 5,12-naphthacenequinone (NAQ), for organic flow batteries, respectively.¹⁹⁹ They found that the redox potential lies between 2.0 and 3.0 V vs. Li/Li⁺ and elucidated that the solubility of quinones is affected by molecular structures, and higher densities of arenes lead to lower solubility. A prototype cell with 1,4-NQ as the active species delivered a capacity retention of over 99.98% per cycle, a coulombic efficiency of nearly 100%, and an energy efficiency of about 88%, and reached an energy density of about 60 W h L⁻¹. Shin et al. synthesized 2-phenyl-1,4-naphthoquinone (PNQ) using a Pd-catalyzed Suzuki cross-coupling reaction and utilized it as a redox-active organic molecule for nonaqueous lithium–organic flow batteries (Fig. 25a).²⁰⁰ The catholyte was prepared by dissolving 0.05 M PNQ in 1.0 M LiTFSI-TEGDME electrolyte. The as-assembled cell presented two distinctive discharge plateaus at ca. 2.4 and 2.6 V vs. Li/Li⁺, delivering specific capacities of 196 mA h g⁻¹ and 161 mA h g⁻¹ at 100 mA g⁻¹ and 2000 mA g⁻¹, respectively (Fig. 25b and c). Moreover, the PNQ catholyte revealed a good capacity retention of 92% over 150 cycles at 100 mA g⁻¹ with a high coulombic efficiency of nearly 100% (Fig. 25d).

In spite of receiving increasing attention, research on organic based RFBs is still at its early stage, and a great number of challenges remain to be addressed. For example, organic molecules are susceptible to degradation reactions such as nucleophilic substitution, gem-diol formation, and self-polymerization, which limits their long-term cycle stability.¹⁹⁸ This has encouraged the development of organic quinone molecules with high stability in both oxidized and reduced redox states. Advanced computational modeling is helpful for the prediction and screening of quinone derivatives for RFB applications. Furthermore, a fundamental understanding of the reaction mechanisms can pave the path towards quinone-based sustainable RFBs.

10. Summary

Redox active organic quinones are potentially low cost, sustainable, and high energy density electrode materials due to their large specific capacity, fast reaction kinetics and excellent electrochemical reversibility. On the one hand, more and more novel quinone structures were developed. On the other hand, the application of quinone-based compounds in various energy storage devices was explored. In the previous section, the relationships between the molecular structures and polar groups of quinone molecules with the corresponding energy density, voltage plateau, and specific capacity properties are elucidated. Generally, quinones with lower molecular weight and an increased number of carbonyl groups can achieve high theoretical capacity. The redox potential of quinones increases in the order of para-quinones < discrete-quinones < ortho-quinones and can be further tuned through judicious incorporation of electron-withdrawing/-donating functionalities. Then the state-of-the-art progress of organic quinones in LIBs, SIBs, PIBs, MIBs, ZIBs, SCs and RFBs is reviewed in detail. It is concluded that many organic quinone molecules and their derivatives, including benzoquinones, anthraquinones, naphthoquinones and other macrocyclic quinones, have been reported as promising electrode candidates. Nevertheless, their practical battery performance is greatly plagued by their strong dissolution issue in electrolyte when used in metal ion batteries and SCs, intrinsic low electrical conductivity and low tap density. To address the aforementioned challenges, we summarized the reported strategies in Fig. 26, which mainly include molecular engineering (i.e. incorporation of specific functionalities), conversion of small quinone molecules into their polymer and/or salt forms, hybridization with conductive carbon (e.g., graphene, CNTs, and porous carbon), the use of different types of electrolytes and/or solid state electrolytes, morphology and phase control, binder modification, etc. With the aforementioned strategies, the electrochemical performance of the quinone-based electrode showed significant improvement, in respect of output voltage, specific capacity, rate capability and cycle stability.

11. Challenges

However, critical challenges still exist for quinone-based compounds before their widespread success can be realized in large scale energy storage devices, and they are summarized as follows:

(1) Organic electrodes are generally not limited by the choice of counter-ions, making them attractive for a variety of energy storage device applications. Although great success has been achieved for organic quinone electrodes in LIBs and SIBs, only a handful of organic quinones have been reported for KIBs, MIBs, ZIBs and SCs. More novel quinone structures suitable for multivalent energy storage devices should be developed.

(2) High performance quinone-based electrodes should be able to simultaneously provide excellent electrical conductivity and high output voltage when serving as a cathode and low output voltage when used as an anode. Quinone structures with intrinsic electrical conductivity and suitable redox potentials are promising for future research.

(3) The immobilization on conductive carbonaceous materials offers an effective strategy to prohibit the dissolution of quinone compounds in metal ion batteries and SCs. Clearly, more novel architectures need to be rationally designed and created to enrich the family of quinone-based composites, and thus further improve the electrochemical performance of the resulting energy storage devices.

(4) Combined experimental observation (both in situ and ex situ) and theoretical calculation provides a valuable strategy to understand the redox mechanism of quinone compounds, particularly quinone compounds with multiple active centers, which opens up new opportunities to facilitate high utilization of active compounds.

(5) Large scale and environmentally friendly production of quinone compounds at low cost is crucial for the application of quinone-based electrodes. However, the development of simple, efficient and large scale synthetic methods remains challenging.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51571144 and 51872157), Shenzhen Technical Plan Project (No. KQJSCX20160226191136, JCYJ20170412170911187 and JCYJ20170817161753629), Guangdong Technical Plan Project (No. 2015TX01N011), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01N111), and Shenzhen Key Laboratory on Safety Research of Power Battery (ZDSYS201707271615073).

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